Because of the decreasing value of older computers and sometimes children leaving the nest and their old computers behind, you could wind up with an older iMac that no longer meets your day-to-day needs, but which you could transform into a monitor.
All the iMacs models that can work in what Apple called Target Display Mode as a secondary display for a modern Mac require an adapter to connect a Thunderbolt 3 port on the iMac to a Mini DisplayPort or Thunderbolt 2 port on the older iMac, and have it function as expected. (Apple appears to have recently removed all mention of Target Display Mode from its site: its former FAQ redirects to an introduction to using Catalina.)
iMacs released from late 2009 through mid-2014 can be used as external displays. Those that followed cannot, starting with the 27-inch 5K late-2014 edition.
Read more at MacWorld.com