What are the purposes and safety to enable the hidden debug menu in Disk Utility?
If you need to ask, you probably don't want to mess! ;)

It's going to let you see things like the Lion Recovery Partition etc that are normally hidden from use.

It's certainly safe to turn the debug menu on.  Whether it's safe to do anything once it is enabled depends very much on your skills.

But it can;t hurt to take a look if you are curious, just remember to turn if off if you don't intend to do anything other than poke about.