Video streaming/on demand is a HUGE bandwidth hog. Peertube exists and has several instances, but maintaining them is expensive. Most give a couble of GB allotment to users, and depending on how much you watch, you can see that fly in a day (or 1h, if any is crazy enough to allow 1440p or larger videos)

With federation, each place has to deal with its own budget. More populated instances are more likely to experiment with ads to help pay the hosting costs, thanks to the bigger audience. For niche stuff, this federation might also prove useful, as you can more easily find and target niche communities, like posting an ad for miniatures on a /wargaming community.