Enforcing a peace is merely an option that you have. I've had to use it a couple times before, when I was still a small nation and getting dogpiled by like 5 different countries because I was low on manpower. When going into a peace deal, it will give you a +10000 acceptance modifier when demanding peace, as long as it's only a white peace. If you demand anything in the peace deal, you will lose the acceptance bonus. Additionally, if the war goal becomes contested (you lose just 1 territory in the province that's being fought over), then you will no longer be able to enforce peace. However, even if the enemy takes a few territories and you push them back, you will be able to enforce peace again (I think there's some timer before it's reset, so you don't want to just ignore it. Take back the province ASAP if they occupy any land there).
It's pretty useful if your enemy is much bigger and could beat you in a war of attrition, or if you have several wars going on at once and just want to get one of them over with. If you're already winning, it's better to just keep going.