There are a number of layers to it.

The most obvious one is grad schools. Some students want to go to grad school and are willing to cheat to make it there. I wouldn't be surprised if much of our political and bureaucratic class are those types of people.

I agree that most employers don't pay attention to grades, but they might use GPA as a screen.

Then, there are things like student aid or various other awards and scholarships that will look at your grades.

Lastly, graduating too many cheaters will have a negative effect on the reputation of your university.