wipes the temporary memory (volatile). A program may have hit a loops (tried to follow instructions, but got lost and is now going in circles looking for a line of code that doesn't exist). If you shut it down, all those processes stop running, and the system starts each program over from scratch so the program knows "where" it is.

Most modern computers have a bunch of software dedicated to checking both software and hardware problems when it starts up. It can fix a lot of these.

Sometimes the issue is the user, not the system. Restarting it gets you back at the beginning of your task, and maybe you'll pay closer attention and get it right this time.

some updates don't apply until after a restart happens - these updates may fix your issue.

it kills all running program. You might have started something in the background that is taking up a ton of resources and causing the problem that you aren't even using. restarting will shut that down and let the programs you want to run have the resources it needs.