Mocks are great for seeing who you should expect to go where.
They are good for guaging where guys like Faulk, Edge, Taylor, Manning, C-Pepp, Vick, Moss, Holt, Harrison, Gonzalez, etc... should be taken.
If, when you are doing your real draft you see any of these guys slip well beyond where they normally get snagged up in a mock, then you know it is a safe bet to go ahead and snag them.
Also, mocks are good for experimenting. In my mocks I try to adopt a different strategy everytime to see how my teams end up. That'll give you a good idea of what strategies work, and what strategies don't.
NEVER EVER take Vick in round 1, though.