Why does restarting a computer fix things?

Basically because anything that's got in a mess gets the chance to start over. Imagine you're making toast and you burn it. Throwing it away and starting again is one way to fix that problem and will always work out better than scraping the burnt bits of toast off.

"Good question!
The short answer is ""it depends""

The longer answer is that Windows has limited resources for applications to use (Memory, Window Handles, File Handles etc.) if a badly written application doesn't give these resources back to Windows when it's finished you Windows run out of resources. This causes problems with other applications.
Obviously the same applies to all other operating systems too"

"Have you ever watched ""The IT Crowd?""

IT support people use ""Try restarting it"" as the first response because:

It often will make the problem go away, at least temporarily.
They don't need to exert any further effort.
They don't need to have face to face contact a human being."

To clean the RAM probably