Why did Stegosaurus have plates?

They were probably used for many things: Thermoregulation, attracting mates, scaring rivals/predators and for a little defense if predators tried to bite its back.

I have a hard time seeing them as used for defense. They were pretty frail and chalky. All other points you make are valid though!

Right. It was the tail spikes that were used for defense, and they could do some serious damage. The plates were used for the tasks Makaveli7 mentioned.

Well they're not fully plates, they come from material that has bone in it, but they're not fully bone, Given they could be used for defence *so they're spine is some what protected form predators* I believe that they were used to help them, they'd act like solar panels and attach/gather some of the suns energy, Help warm the blood,*everybody likes to be warm :D* OR something like that... They wouldn't have grown them for no reason... nature has shown this before you evolve these things for a reason... we need to find out what that reason is

Right. I think that this could also help prove that some species were cold blooded.

I think they were used for thermo-regulation. They could catch the suns rays or a cool breeze. They probably were full of blood. This meant they could have been for display as well. They were not for defense in the sense that they kept a predator from biting them. Say an Allosaurus bites one and breaks it. It would be like a big, blood filled cookie for him. The only way they could have been for defense is if they could change colors. That could frighten off some predators. Kind of like in Walking With Dinosaurs.