My dogs do it because it is step one in their plan for world domination, mwah ha ha! (That was their evil laugh.)
Fortunately step one is so comfy it seems to have distracted them and the world is once more safe.
True fact.
(SlideIn I hope you don't take offence at my gentle ribbing, dominance theory has been completely discredited, but that doesn't discount that some dog's will take a mile if you give them an inch so keeping them off furniture can be a element in that. I tried it, along with the doorway and eating first thing with my bossy bitch but it made no difference for her.)