Aaaar thats so cute! like the others have said, probably a combination of 'look at me', the warmth and the high vantage point makes it a good spot to perch.
When I leave me sewing chair and come back to it, usually one of my cats is sitting on the seat, maybe because of the warmth left by my derrier (!) , but they are always staring at the sewing machine as if to say 'why are you getting all the attention!?
They also jump on my lap and wave their bum/tail in my face if I talk on the phone, and walk back and forth across the keyboard when I am working on my laptop.
LOVE 'EM!!