Mine do that too!! Cats like to be the center of attention I think...well mine do! If You're focusing your attention elsewhere, they don't like it!! Mine sits all over my laptop too if I'm trying to use it!!! They're a pain sometimes...but I love them!!!
My cats the same. She's cheeky, if you get out of the chair to close the curtains or something she goes and sits on it. And sits on magazines on the floor etc etc...
It's not just cats, Max my GSD is quite happy to lie about even while I'm using my desk top but if I get my lap top out. & use it on my knee he comes and puts his head across the key board and stops me using it, if I'm at my desk he sits & paws me til I put it away. He doesn't bother about anything else just my lap top
As the others have said its all about attention - if you're reading it means you're not paying attention to her and so she tries to remedy that by sitting on what you're reading!
Mine does it too. If I'm reading he headbutts the book out of my hands, if I'm watching tv he stands on me with his face directly in front of mine so I can't see the screen, if I'm working on the computer he walks about on the keyboard.
Basically what they're saying is "look at me look at me - me me me"!!