How can the game be fun if it isn't challenging?
There are several types of challenging. The two most important ones are:
1. Exciting-Challenging
2. Boring-Challenging
The only cheats I use myself, is to force peace deals that make sense (such as when I and my Muscovite ally occupied all of the PLC and were standing strong - I should be able to demand pretty much anything at that point since I have completely occupied every last province, slaughtered every last soldier and supressed any rebellion while not being exhausted at all from the war) and maximize my stability when it makes no sense that I've taken a hit, such as when you want to declare war on Riga. Why should Sweden, who traditionally held a great interet in the Baltic, suffer several national uprisings because I want to annex Riga? makes no sense.
Thus, there's a difference between cheating because you can and cheating because it improves the experience. I generally try to limit cheating to a minimum, it's when I just can't be bothered suffering a bunch of ping-pong rebellions, or a stability drop makes no sense like I've already covered, that I go to the console and type 'stability'.
Cheating can also be a great way to ''roleplay'' your particular game, as well as in order to represent events. It can also be done in the favor of AI nations.