Why does deleting a program cache speed it up?

[DELTAsnake]

World of Warcraft was taking 5 to 10 minutes to load, I had no idea what the problem was then a Guildie suggested deleting the cache files in my library folder. I deleted them and miraculously everything was fixed.

So why does deleting cache files speed up a program? I would have though it would slow down or even break a program.

 

[sth]

That's a WoW-specific problem. Seems like the files had been screwed up.