What's a tune-up? Since technology changes at the speed of light, your computer needs to be updated to keep pace. Without these updates, you can go to a website and videos won't play (or play correctly), and you may get notices that you don't have the needed software to view certain pages. You may also be unable to open some attachments that you receive with emails.
Why should a computer need a tune-up at least once a year? Stuff happens. In the computer world all sorts of things happen. Files get corrupted. (I asked Google why and got 1.2 million answers.) This can happen for some pretty obvious reasons. Power failures, power surges, program crashes, computer crashes (yes, even Macs). It can also happen for no reason at all. I like the cosmic ray theory, but any theory will do.
Files get corrupted. These can be unimportant files or crucial files which bring the computer to its knees. When it is the latter, you don't need a tune-up, you need a troubleshooting service call. But even when things are humming along, there are generally minor problems lurking under the hood. And speaking of cars, here is the analogy: used cars are never in perfect running order. In cars, parts wear out. In computers, files get corrupted.
Diagnose and Repair. The good news is that most corruption can be repaired with special programs. Some of these programs are safe to use. Some are risky. I don't use the risky ones. As part of a tune-up I identify corrupted files and repair them when possible. Occasionally this uncovers problems that have been affecting your computer for a while. That may mean some applications may need to be re-installed or the operating system updated.