I'm looking to upgrade to Adobe Photoshop 2018 to 2021 and have read that multiple versions are allowed on the same machine. Why would you want to have different versions on the same machine, and what are the advantages/disadvantages of doing so? Tech support was not helpful in answering this question.
There could be a number of subjective opinions based on the requirement of the end user. Photoshop Elements 2021 is definitely much more refined in terms of features that may checkout here
Multiple versions of Photoshop Elements are kept because sometimes users might need some time to switch to a newer version.
Some feature/tool usage works better in the previous version for the user.
Time might be needed to migrate over to the new version
The only down side could be that some extra storage space will be occupied. However, previous version can be removed anytime once the user feels that everything has been migrated and they do not feel uncomfortable with the new version.