Groups is just a way to group similar accounts...
There are many reasons to use custom accounts...
1. Multiple logins at the same URL - ie, you have more than one gmail or yahoo account, so you set up an account for each (each with its own username)...
2. Account has special requirements - ie, won't use certain characters in password, or has password size restriction, or you want to use different settings for some other reason...
3. You just like to be specific and are organized...
Yes, the Export/Import can be use for backing up - but currently all you really need to do is copy the passwordmaker.rdf file to back up your settings...