Normally, whenever a new OS update is available, I just install over top of the current installation, unless that it isn't possible. If one is pressed for time, following this approach can be a great benefit.
However, there are also times when a clean install can be beneficial.
Look at this way: Over time, particularly over years, as you continue to use and upgrade your OS, add new apps to it, remove old apps from it, etc., you slowly build up unnecessary clutter in different areas of your hard drive. You can have duplicate files that you are no longer aware of. You can have orphaned prefs files and other files and folders that belong to apps that you no longer even have on your hard drive. You can have corrupted files. You can have settings that got messed up at some point.
A lot of things can happen over time. Your computer can become like a messy attic or cellar. The end result is that it can affect your memory usage, your hard drive's seek time, how long it takes for an app to open, etc.
There are many apps out there which will address the aforementioned issues to one degree or another. I use some of them myself. However, none of those apps is 100% perfect or catches everything, despite their claims.
If your computer seems to be dragging along, gets a lot of beach balls, freeze-ups, etc., then maybe it is time to conduct a clean install. It is like taking a much-needed shower.
As Simon noted above, a key point to remember is to
always backup your current startup system first, before you undertake a clean install. My personal habit is to make two full daily backups of my internal hard drive to two external USB hard drives, using Carbon Copy Cloner. That way, if I ever mess up anything seriously on my startup drive, I have two reliable full backups.
Once you have backed up your startup drive, go for it. Do a clean install. Then, use the Migration Assistant to copy over all of your personal user files and folders from your backup.
Yes, this does require a considerable amount of time on your part. However, if your startup drive is in a seriously clogged up mode, you will notice some level of difference after conducting a clean install.