If 1000 people hated you online or something, turning the PC off and walking away doesn't change that. You're left with the knowledge that people still hate you, and I know it's crazy, but we're all human.
People say vile shit to each other, especially when there's no identity attached. Sure, you can't see or hear these people physically, but they still exist. Imagine walking into a crowd of people who just start shitting on you for, well, I dunno, existing?
Point is, it gets to some people, especially if you have self esteem issues. And turning off a PC won't change that knowledge, and for some, that weighs on them.