Because you want the best accuracy, at the desired velocity, from your gun.
The first law of accuracy is uniformity. Uniformity includes a good gun, your aiming, holding, and squeezing precisely the same shot after shot, and proper ammo where each round is exactly like the next.
If your ammo shoots to one hole, you know it is uniform. If your ammo leaves something to be desired, a good start is making sure that its velocity is uniform. With a chronograph, you can easily measure that the velocities are uniform, and that the average velocity is where you want it.
We can’t guarantee that ammo with the most uniform velocities will always shoot the best groups or even shoot good groups. We know it’s a good starting place. If you know the uniformity and the average velocity of each tested load, you can find the best load faster and easier.