I've always been curious about Youtube networks. I regularly have them reach out to me, most are form letters so I know a million people are getting them, occasionally I'll get a personalized invite but like you said I've never seen what value they bring.
From my understanding it seems like all they do is take a cut of your Adsense revenue and in return maybe give you some royalty free music to use, just doesn't seem worth it giving them a pretty big chunk of your royalties for some music you can find free or buy for a much smaller amount.
One thing where I think networks may be valuable is for gamers, again my understanding of this is limited but I think sometimes they can get the rights for gamers to monetize certain games they may not be able to monetize on their own.
I've always been curious to learn more about how they work.