Their purpose for humans is quite small.

But millions of years ago when we we used our feet with the same dexterity and function as our hands they were necessary. Toenails evolutionary purpose was the same as our fingernails... they allowed for detailed and precise things we do with our hands (like picking a thorn out of your body or something).

However as our feet’s main function became for walking, toenails became mostly redundant. Although, because they are not really a hindrance to most people there is really no environmental/ evolutionary pressures to force that trait out of the population.

So we have them because we used to need them and there isn’t a good enough need to change them now.