using the -AsHashTable and -AsString parameters with Group-Object cmdlet

Hello 

I have seen a few examples (cannot lay my hands on one now unfortunately) where people has used -AsHashTable and -AsString with the Group-Object cmdlet

can someone place explain a bit more about these two parameters e.g. do they always need to be used together or can they be used separately

can anyone give me some examples of when and where these two parameters would come in handy e.g. when and why to use

Thanks

AAnotherUser__