It simplifies and clarifies without burden, enabling you to think about your work.
I find them useful for to-do lists, but little else. Even if you can connect two separate nodes, the tool doesn't use that new information to change the organization of the nodes (something like a force-directed graph could be interesting).
Scapple just lets you draw nodes and edges. I fell in love with it a while ago and use it to collect my thoughts, and to put together project plans (treating it like a dependency tree, even though it doesn't impose that constraint on you)