I've heard several explanations for this through the years.
1. For reach shots you can but the butt down on the table and sort of push it, instead of using an extension. Harder to explain than to show on the table. You are still using the tip to strike the ball but the bevel is touching the cloth.
2. Some people claim that originally the bevel was there to be able to reverse the cue and use the butt like a mace.
3. At the present the bevel gives the player a reference point so that they can easily orient the cue the same way for every shot. Especially useful if your cue is slightly crooked. If you grip near the bevel you will also receive feedback if you have various stroke flaws etc.
4. The bevel makes it possible to have the manufacturers mark on the cue and improves looks.