It is the oldest line in IT, and people hear it as "go away". It is not. Restarting fixes a surprising share of what we are asked about, and there is a proper reason why.
In short
A computer that has been on for weeks is carrying weeks of small mistakes. A restart clears them.
Closing the lid is not a restart. Neither is logging out.
If the same fault comes back after a restart, that is useful. It tells us the fault is real.
What builds up
Every app you open borrows some memory and is meant to hand it back. Most do, mostly. Some keep a little each time. Background helpers get stuck halfway through a job and sit there waiting. The connection to the printer or the Wi-Fi gets confused and never quite recovers. None of it is dramatic. It adds up, like crumbs in a keyboard.
What a restart does
It throws all of that away and starts again from a known good state. Every app is closed, the memory is emptied, every background helper starts fresh and every network connection is made again from scratch. Any update that was quietly waiting for a restart finally gets to finish.
Sleep is not off
Closing the lid puts a Mac to sleep. Everything is frozen exactly as it was, stuck bits included. Plenty of the laptops we see have not properly restarted in two or three months.
You can check your own. Hold the Option key, click the Apple menu and choose System Information. Under Software, "Time since boot" tells you how long it has been.
When it will not help
A failing disk or battery. A restart hides it for an hour, then it is back.
A problem with your account, not your computer. A password, a licence, a full mailbox.
A fault that returns every time. Restart once. If it comes back, tell us exactly what you were doing when it happened. That pattern is the clue.
How often
Once a week is plenty. Friday on the way out is the easy habit: updates install over the weekend and Monday starts clean. The same goes for the printer and your phone. Ask before you restart the router in a busy office, though. Everybody's call drops at once.
Restarted and it is still broken?
Then it is a real one, and you have already done the first thing we would have asked. Talk to us and tell us what happened. If you would rather it was found before you noticed, that is what managed IT services is for.