I am going to talk from the basis of gaming, there are many other partnership programs, but this is the one I know about.
Basically on YouTube, as everyone knows, you can get partnered and earn money. However, YouTube's adsense works on the basis of ads clicked on, so if nobody clicks on your ads, you won't get paid, plus you only get paid every $10 you make, which seems easy enough, but you only get about 10 cents for a click, if that.
Another problem is that if your channel gets big, and you are with the YouTube partnership program, if you have any strikes against your account for copyright, which you would get for trying to monetize gaming videos, then you could get banned, which would mean you would have to start from the very beginning again.
Now, if you get with a partnership program eg. fullscreen, machinima, TGN etc, you will get a set pay per CPM (Cost Per Mille), and this will range from between $0.92 CPM to $3.00 CPM, and possibly even more if you get really big. so you get paid a fixed rate per 1000 views, which is a lot better way of making any money from your videos.
Also, most, if not all partnership networks have paid licences to all the different companies for copyright liabilities, and will mean that for as long as you are partnered with that network, you can use their content and make money off of it without any copyright strikes or claims being made against you.
That is the main reasons for choosing a network over YouTube, hope I cleared that up, feel free to ask any more questions