The whole point of your CF rating is to reflect your past performance in contests, which gives you a rough idea of how skilled you are as a competitive programmer.

So when your rating goes up significantly, it's usually a sign that you have improved significantly.

I would assume that people cheat for a better rating, but what is the point of increasing your rating if you know your skill hasn't increased alongside it?

It makes your rating feel meaningless... Completely removes the sense of accomplishment, which is the whole point of wanting to achieve a higher rating in the first place.