Well, in C++, "const" isn't actually intended to mean "literally const", it's supposed to mean "const in spirit". You can be const and still change some of the bits of an object if that change doesn't (semantically) change the object. To be able to do that, you need to be able to cast away const-ness. (No, I can't think of a reasonable example at the moment.)

It would be fair to ask whether that intent was a good idea. It would also be fair to point out that many (most) times people cast away const-ness simply out of expediency, rather than using it the way it was intended.