Snapshots are an amazing feature for testing purposes.
Lets say you want to test out a new freeware app that you found, and you're being a careful, smart geek, so you decide to use a virtual machine to test out that freeware app that might be sketchy. Great.. but what happens if it is actually crapware?
Nobody wants to have to reload a virtual machine all the time.
That's where Snapshots come in - you just take a snapshot of the virtual machine, test out the software, and if you don't want to keep it installed, roll back the snapshot. Easy. Simple.. and safe.
If you aren't using snapshots today, you should start right away.