Kids have to at least go through a phase where they develop their own identity (and not everyone does go through that, even then). But most children and teens still have their values tied to an idea of how they believe their idols/parents/friends and so on perceive it, it's an active part of brain development to decouple yourself from that and stop thinking about your decisions through a lens of how you think other people think about your decisions/likes/thoughts.

It makes an account look authentic, which makes recommendations that account offers more genuine. E.g. advertising, political astroturfing.

There's so much of that on Reddit with accounts that look real but aren't. When I started using reddit (like 15 years ago) I was legitimately able to trust most recommendations (I still own and use many of the products I picked up and consider them good decisions), but it's been at least 7 or 8 years where you absolutely can't.

If I remember correctly, when it first launched, it was a one time payment actually.

I'm on both, but I'll stop using reddit when I can't use sync anymore.