It doesn't matter how they work.
What matters is what users want to do.
The interface should not be designed according to the machine, the interface should be made according to the user. Doing otherwise is bad design.
The fact is the most common thing people do with snapshots is say "Ok I like the VM like this", and they write a whole description of the snapshot. but very often right after that they make another change and say "Ok... NOW its perfect".
Instead of having to take 3 steps
* Open Snapshot Manager
* Copy snapshot description
* Memorize title
* Delete Snapshot
* Wait a for it to finish
* If doing other things open notepad and paste description into notepad
* Check that it has finished deleting the snapshot
* create a new snapshot, remember the title
* copy old description from notepad
* paste the old description in
* click create
* close snapshot manager
vs the alternative (ammend the snapshot menu)
VM > Snapshots > Shapshot x > Restore
VM > Snapshots > Shapshot x > Overwrite
It doesn't matter what happens behind the scenes, the end result is the same, only much less hassle for the user to go through.