Mega myth #1- To 'cleanse' your body!
Sexist societal taboos wrongly position periods as 'dirty' and sadly, this is what many people continue to be taught to believe . For nearly a century we have known for sure that menstrual fluid does not typically contain any toxins, diseases, or 'bad' bacteria, and yet the myth that menstruation is a type of 'cleansing process' persists .
While disposable period products can encourage bacterial growth (leading to a sweat-like smell) menstrual fluid itself simply smells like blood (a metallic odour) plus our unique body odour, and occasionally garlic or coffee ! [Remember- a fishy smell indicates a common (non-sexually transmitted) vaginal infection- Bacterial Vaginosis].
It is true that menstrual fluid can stain clothing, rendering it visible and inconvenient, but the same could be said of semen, ink, blood from elsewhere on the body, spaghetti sauce, or baby vomit, and yet societies do not tend to freak out about the possibility of these stains, at least not in the same way that they do about period leaks... This reveals a near-universal irrational fear of menstrual fluid and, by association, the bodies and social identities of those who menstruate .
Even supposedly 'empowering' menstrual health education practices can reinforce this damaging belief. I've lost count of the number of self-help books/ seminars/ resources that imply or directly state that the period is part of a physical/ emotional 'detox' or 'spiritually cleansing' process . While the intention of these practices may be good, such beliefs reinforce the inaccurate and sexist positioning of the female (reproductive) body as inherently impure.
Mega myth #2- To 'prepare for pregnancy'
This is probably the most common misconception regarding the purpose of periods . Due to inadequate menstrual health education (in school and even clinical training), most people are left with an inaccurate impression that menstruation must have something to do with getting the womb ready for implantation -when the fertilised egg first embeds in the thick and nutritious lining of the womb.
Of course, if you think about it, it's unlikely that the shedding of the uterine lining from the body (as occurs during a period) is helpful for establishing pregnancy. In fact, it suggests almost the opposite purpose (see the two evidence-based theories below).
We also know for a fact that menstruation has nothing to do with 'preparing for pregnancy', because only relatively few mammals menstruate, and yet all mammals are able to reproduce . What is more, menstruation seems to have evolved over time, in at least four separate genetic lines within the 'mammal' family tree, each from ancestors who did not menstruate . This suggests that menstruation is a specific adaptation to environmental pressures, rather than a universal process required for pregnancy/ fertility .
Evidence-based answer: Due to 'spontaneous decidualisation'
What do all of the menstruating species of mammal (approx. 84 out of 5149 - 1.6%)  have in common besides periods?
Well, interestingly, there is something unique about us and that is a super cool thing called 'spontaneous decidualisation' ! This is a complex process, but in short, it specifically refers to a change in cell type from 'endometrial fibroblast' the typical cell found in the lining of the womb, into a 'decidual stromal cell' or DSC . These DSCs are the initial starting block cells responsible for creating all of the different types of cells required to build a new human being.
In most mammals, this process only happens once a fertilized egg successfully enters the womb. However, in humans and all other menstruating species (including some apes, bats, a mouse and an elephant shrew!) this change occurs when progesterone levels fall, even if there is no embryo or unfertilised egg present ! Note- This is also how 'withdrawal bleeds' are induced in people taking contraceptive medications (i.e. when they stop taking the active pills for a few days).
So, why did spontaneous decidualisation evolve?
Evidence-based theory #1- To save our lives
Although this theory is a bit tricky to test in practice , it is well supported by observable patterns in mammalian characteristics and behaviours.
For example, species vary in respect to how invasive the placenta is within the womb. [The placenta is a temporary organ developed at the same time as the embryo to facilitate the exchange of nutrients, gas and waste between the different (sometimes incompatible) parent and offspring circulatory systems] . In some cases, especially humans and other Great Apes, the placenta is extremely invasive and can embed too deeply into the womb, which causes extremely heavy bleeding and poses a risk to the life of the parent .
Also, one of the roles of the decidual cells is to regulate the depth of the placental embeddedness in the womb, which again implies a direct relationship between 'spontaneous decidualisation' and 'placental invasiveness' . It could be that spontaneous decidualisation helps to prevent more cases of dangerous placental invasiveness, reducing the risk of maternal death.
FYI- before modern medical practices significantly reduced the maternal mortality rate in the UK, approx. 22% of deaths were caused by placenta-related haemorrhages . So, even with the evolution of spontaneous decidualisation, human reproduction remains highly risky compared to most other mammalian species.
Evidence-based theory #2- To test the viability of the embryo
Possibly 'as well as' rather than 'instead of' the first theory, is the idea that spontaneous decidualisation occurs as an automatic 'test' of the quality of an embryo before pregnancy is fully established . The decidual cells have been observed to react (a stress response) to poor quality embryo tissue , which suggests that the cells may play a role in whether or not an embryo successfully implants into the lining of the womb.
This theory is supported by the fact that humans also have a relatively high rate of pregnancy loss after implantation (10-25%) , a high rate of maternal mortality (before modern medical practices were introduced) , and the fact that nearly all menstruating species have relatively few offspring per birth . In other words, the high level of physical (and potentially fatal) investment per offspring is extremely risky. Spontaneous decidualisation may therefore reduce the risk of dangerous, or simply inviable, pregnancies progressing to the point at which maternal/ infant mortality is highly likely.
In conclusion, while periods are fairly costly in terms of the loss of iron stores (most females of reproductive age are iron deficient/ anaemic) , uncomfortable/ painful, and often disruptive, they ultimately protect our lives and enable our species to survive. That's a pretty cool purpose and one worth sharing with children, parents, teachers, and doctors in order to improve health, self-esteem, and to smash those stupid sexist myths!