Okay so heres how it how it works:
1. Companys pay for ads on your videos.
2. YouTube takes 45% of that money
3. You get 55% of that money, if you become a partner
4. If you join a network they take a % from your 55%.
For example a 90-10 split they would take 10% of your 55%.
And a 80-20 split they take 20% of your 55%.
The reason why you deal let the networks take your money is so you can get features such as payment via paypal, lower minimum payouts audio music libraries, promotion and shout outs, support and help and more.
You can make money on your videos without joining a network, but you have to have a bank account and you won't get paid until you get $100. So some people think the features networks have are worth giving up a portion of their money.