A 27" Late 2015 iMac is still a very nice machine. Although you can't update it past Monterey – and are therefore locked out of current Adobe and Microsoft applications – Monterey is enough to run the current versions of
- Firefox
- LibreOffice
- The GNU Image Manipulation Program (GIMP)
- The commercial Affinity V2 applications (Photos, Designer, Publisher)
The machine has USB 3.0 ports and can take up to 64 GB of RAM.
If the Mac is in good condition, and you don't want to use it any more, you could perhaps sell it, and put the cash towards the purchase of a standalone hardware monitor for a new Mac. Or donate it to a non-profit organization who could use it, with the above-listed applications, for general office purposes.
If you do this, be sure to prepare the system properly for a new owner.
What to do before you sell, give away, trade in, or recycle your Mac - Apple Support