Discovering new things related to something I just watched or combing through a specific topic - just two things that I can immediately think of.

An aggregator is great when you dig yourself into a new topic. For instance, say you want to learn more about Speedrunning. Youtube's algo can aggregate a lot of speedrun videos for you and you can chose where to start digging the newfound rabbit hole. x)

This is, unironically, a great feature and why I think an aggregator has it's merit.