By narrowing down the possible events it makes teaching and drilling the defensive linemen easier. The defensive linemen know the possible scenarios that they will be reacting to on every play.
Defensive linemen, especially tackles, start every play in the worst position on the field. They do not know when the ball will be snapped, they can be hit by multiple different players in a variety of ways and they have to make a play on the ball right away.
This means that their only real chance at being successful is to limit their assignment and do the assignment as quick as possible.
