Why would someone blanket, say, a yearling, 2yo, 3yo? For the very reasons I and a few others already stated.
In the Fall, before coats are really in, it’s not uncommon to have a wet cold spell with a temp drop of 20* or more (F) and rain for a whole day/night, and this is usually because of a cold front blasting through, so it’s usually quite windy as well. Going from mid-60’s or so and sunny, to mid/lower 40’s or so and rain and wind with a partial Winter coat makes a lot of horses downright cold.
In the Spring when a good bit of the coat is shed and not even an equivalent amount of new coat is in, the exact same thing usually happens. It makes many horses flat out cold.
The same conditions in the middle of real Winter leaves horses much better equipped to deal with it. But even then, we often get a dramatic worsening of weather for a few days, temps well below normal, winds. I will not let even a yearling be outside in 33* weather with 20mph winds and heavy rain for 24-36 hours. But I’d also really really like to keep them out, so on goes the blanket.
No harm, no foul, everyone is happy.