I posted a vent about this last summer - since it was my workplace that required it, numerous times a frickin’ day. The upshot was these are being used to help train self-driving cars.
I have to assume there’s some money changing hands somewhere, from Google (or whatever) to those sites who’ve been persuaded to implement them.
As a fun side note: there are a lot of rumors out there that fun games on Facebook like “your porn name is your first pet’s name, plus the street you lived on at age 8” are ways for ID-theft harvesters to get that kind of info.