I believe it's a 'I demand your undivided attention' thing. But it's only when they feel like it.

If I am doing something critical on the computer (like playing a game lol where random stops will ruin it) Tinkerbelle always thinks that's the best time to get up on the desk, and stand right smack in front of the monitor...and will not move until I give up and move her myself.

They do it just because they can.