It doesn't matter what scheme an offense runs because identifying the MIKE is very important.
AGAIN, the QB is identifying the MIKE because that sets up the entire blocking scheme. With formation shifts and variations in alignments, it's not always blatantly obvious who the MIKE is.
Teams don't intentionally just leave LBs unblocked.
Even zone blocking teams don't use zone blocking exclusively. And even in zone blocking, establishing the MIKE and alignment is very important because it determines what "zone" you have as well as the need to combo blocks and reach blocks.
Any team that I played for or coached or was associated with had a different term they used for MIKE (the free man). We still identify the MIKE every play so as to not make it obvious when you were running it or passing it.
It's important to note that when the QB calls out the MIKE, it is not necessarily what the defense considers the MIKE, nor is it necessarily even the MLB. When the QB calls out the MIKE, it is typical that the guy who is being singled out will get a lot of attention on that play (maybe they will run right at him, or away from him).
In passing plays, the MIKE is called out similarly for pass blocking reasons. Often when a team is in a max-protect scheme, the MIKE is called out to the RBs to let them know that they need to block him if he blitzes. The reason that this is important is that if the MIKE LB does not blitz, then the RB is often free to run a short route into the flat.
Also, the QB points to the MIKE for the benefit of the skill position guys who are looking for the snap of the ball. When he points the MIKE out it points to their blocking assignments. The center is normally responsible for making blocking assignment calls at the line for the OL. Thats why you see the center pointing and talking before the snap as well.