When my father was a child, there was a craze in town (India) that tonsils are bad and removing them will only do good. My grandma fell for it, among hordes of other mothers, and got my fathers tonsils removed. My father is a Gen X cohort, and now whenever the pollen season arrives (he is in USA now), he is totally helpless.
Despite having had repeated tonsillitis as a kid (and bronchitis at the same time - unheated boarding school with abysmal hygiene and dozens of boys to a room is quite the disease incubator), my mother, a nurse, dug her heels in, and said no. She'd had to deal first-hand with the outcomes of botched tonsillectomies a few times too many.
They did my brother seven years later without their permission - and he suffers from severe allergies to this day. I have none.
I’ve always been extremely sceptical of “its vestigial” or “that body part is bad for you”. I mean, surely if either were true, selection would have done away with it. If having tonsils were a dysgenic trait, nobody would have them.
I can tell you why it remains such persistent practice - it’s a quick and easy surgery in most cases, and it’s billable at a decent rate in most places. A surgeon in the UK can do ten or more in a day, at £3k a pop. You have an endless supply of patients, and parents who’ll consent because after all they had theirs done when they were kids.
Play your cards right and you’ll make millions a year.