Is it necessary to turn off the iMac after use?
It is necessary to turn off the Imac after use, or leave it on constantly.
[Re-Titled by Moderator]
For the past 20 years I normally leave mine on 24/7 and do a reboot about 1x every 2 weeks. It's really up to you though, if you want to schedule shutdowns and boots Mac OS has that ability in Energy Saver. Here is an article that explains it:
https://osxdaily.com/2021/03/01/how-make-mac-turn-on-off-automatically/