They're pretty much like someone's personal 'signature' on a save - no real practical purpose or meaning, though there are some group seals - such as the Brotherhood of Saves, for example.
Sometimes they may serve a function as they may be related to the save through appearance, function or containing electronics that run the save.
Wait a sec I just got a great idea! What about using the electronics that run a save as the logo? That way anyone who tries to steal it will either have to remake the electronics or still have another person's logo in the save!
Great idea unless they delete the deco layer on your logo and add their own.but then again you can use just nomal elements to make the logo so they will have to do everything over like you suggested.