The guru is a personification of unity/god/the flow/silence. For some people, it's helpful to have a "local" version of all that. The pitfall is getting stuck on the local version and never jumping to the universality of it all (or, per this discussion title, having your local personification grab at the wordly perquisites of deity-hood). But the thought is that it's easier to surrender in two small jumps (to local guru, then eventually to the universal) than in one. I'm more of a one big jumper, I think.
It's sort of like driving an automatic car that drives itself, but you are, insanely, sitting at the wheel, very tense, very worried, very stressed, certain you're driving the damned thing (feeling bad about wrong turns taken and rear view mirrors accidentally knocked off, and feeling proud when you're really whipping around corners slickly and making good time). Getting a guru is like hiring a chauffeur to take over "driving" duties. You learn the valuable lesson that you don't have to drive, but you still haven't figured it all out (but at least you can relax and get to work on figuring out the rest). The chauffeurs - even the bad evil ones - mostly all understand the truth behind it all. But sometimes they get caught in the same delusion and revel in the driving, and stop the car to buy blow and hookers.