[mimetist]

Reformulating your question: Why sponsor a football team when you can just buy ads on TV?

Advertising and Marketing is (mostly) a way of making your brand more visible to possible customers. Sponsoring a trendy youtuber you get more visibility than other companies that just bought some ads.

You also have to take into account the friendliness of your advertising. Some ads might become annoying just because you saw them a few times in a row... sponsoring allow brands to buy you a "friend" that tells you how great they are.