I guess the main reason here isn't "keeping an old version", but having a version that doesn't require an internet connection to be activated and doesn't send any data to Adobe.

But having an older version can be useful too because some features from previous releases may be missing in current ones, so that's a way to ensure access to the old files. A couple of years ago all the Pantone colours used in Photoshop just became black after an update because Adobe stopped licensing Pantone stuff.