It's not about that demo having money, it's about that demo being impressionable. If sports clubs get them while they're young, it creates a nostalgia that will get them coming back for more. For advertisers, it allows them an opportunity to create favorable impressions with them from the start. Then when they get money, they'll be inclined to use their product.
So say you're an Orlando fan. You don't care much about Orlando Health, the sponsor for Orlando SC. You might not care now as a millennial, but when suddenly your wife/girlfriend shows you a stick with a plus sign on it, guess what the first place that will pop into your mind to go will be? Orlando Health? Now you're in their door forming a habit with them, and of the money you earned cooking meth (this is an Orlando fan in this hypothetical after all) devoted to healthcare will be going to Orlando Health.