It varies from network to network but they will usually offer you: An Increased CPM (however they take their cut so this doesn't always = more money), Quicker support than YouTube alone, instant monetization (you won't have to wait for YouTube to review your videos before monetizing), Depending on the network some will offer extra tools such as a Library of Music, Promotion (very rare in most cases), a Hub channel for you to upload too (directorship program), a referrals program (getting paid for referring other partners to their program), Channel Branding (Some networks will offer you custom made branding but this varies on how big a partner you are), Some offer their bigger partners custom made websites etc.
It really greatly depends on which network you go with and how much leverage you have (in terms of views per day) to negotiate yourself a good deal.