What really gets me thinking is why a Lion Guard in first place, like isn't the King supposed to be in a way a Lion Guard? the male lion leader is supposed to protect its pride, so I don't see the point of a guard...
But now about this post, I just think that supposing being a guard is such a great responsability and that one lion would be supposed to need some extra-help for this extensive task, like if a lion guard is to protect everybody, and it's supposed to do it alone it must have emerged from the need to be more powerful, and since the King would have different tasks, I guess that's why the roaring thing is given to the guard instead, because he'll be the one needing that aid.
But then again I will have to see why would it have ended with Scar, or why Simba didn't have it as he was only cub... Because hey I can understand it didn't continue with Simba because Mufasa wasn't there to give Simba a little sinbling to fulfill that place.