my iMac is asking me to reinstall OS X. How and why?

This discussion was somehow affiliated with an Apple Support

article precisely on topic; with several good ideas to try & use.

• How to reinstall macOS - Apple Support

https://support.apple.com/en-us/HT204904

Some secondary links within this, tell of Other macOS options

to use, as those may be better suited in your Mac.

If you happen to have backups from Time Machine, w/ external

USB hard drive; that's a clue to search to learn more about this.

'Erase and install' may provide different system than you expect.

The most recent or latest macOS may not be the best one to install.

Especially should your Mac not support the features, or if your fav

third-party apps, run in 32-bit while later Catalina+ runs 64-bit only.

Hello, good question! This could be caused by a corrupt version of OS X. Try restarting your mac, and if that does not work, try reinstalling the Mac OS version. To reinstall OS X, head to the app store and search Mac OS Big Sur. (assuming you're running the latest software version already) *Always make sure to backup your files before reinstalling the software in the case that a file goes missing.*

An iMac is not a laptop.