This is important because Qbs in the league rarely have a perfect pocket where they can just sit there and wait for the play to develop. This means that a quarterback must thread the needle and throw in between linemen at the line of scrimmage. This is why you see the great quarterbacks like Patrick Mahomes or Aaron Rodgers using the side-arm technique to be able to fit the football in between those tall guys.
Here is a very good example from yesterday's practice where you can see QB Justin Fields making a great throw right on target to WR Darnell Mooney.
Here are a few examples from Patrick Mahomes. You can see where the "side-arm" throw can come in handy.