why are installed apps showing up to reinstall?

Do you have your Desktop and Documents folders using iCloud?

Add your Desktop and Documents files to iCloud Drive - Apple Support

Anything added to the Desktop of another Mac using the same Apple Account will also show up on your Mac. That is one of the features of using Desktop/Documents in iCloud and you can also turn it off using the link above. You can check if you have another Mac using your Apple Account by going to  > System Settings > [name on top], and scroll to the bottom to see the list of devices.

Since they are already on your Desktop in iCloud, moving them to your Desktop folder will do nothing because they are already there. Drag them to the trash as they are just the installers that were initially used to install them in your Applications folder.

MABM wrote:

All of a sudden several apps have shown up on my desktop that are already installed. But they are not downloads that one gets when one buys a new app, they are the things you get after you have clicked on the download. I'll include a screenshot of three of them. I use them once in a while, but not always regularly. But when I try to eject them, they don't go away. If I try to move them to a folder on my desktop, they will not. They just sit there and if I click them, they invite me to install them. They are already installed!!!!

They look like version update installers for those app's.

Control (right) click and select Get Info on one of those and the corresponding app currently in your Applications folder. If it is an updated version of that app, then you need to open and install the new version.

Then once you have used the installer to update the app, you should be able to eject and/or trash the installers icon.

What App's..?

Are they trying to reinstall or update..?

App Store app's like Pages, Numbers and Keynotes need to be updated after a macOS Update.

How to manually update apps from the App Store - Apple Support

If you did an Erase and Reinstall, then Pages, Numbers, Keynotes and third-party App's need to be reinstalled.

CCleaner should NOT be re-installed, it should be removed.

Your exceptionally well-crafted Macintosh computer does not accumulate filth that needs any third-party anything to clean it. Everything needed to run it efficiently was included in the box, except ONE: a drive on which to store a second copy of your files in case the first copy is damaged or deleted by accident. The default backup software, Time Machine, is already present -- integrated deeply into MacOS.

You may need to provide more details on what you are referring to by less used apps or what you are actually seeing. An app will update itself when one there is an update available, no matter if you are continuing to use it or not. You can delete third party apps not installed as part of Mac OS.