Range is important if you fire from further away than 400m
Even at point blank ranges, having a precise range lets you send precise shots.
The reason being, at longer ranges, the gyro angle of the torpedo must be greater, since the torpedo has to lead the target. So if (for example) you estimate too short a range, the torp will end up hitting the aft of the ship instead of the sweet spot you were aiming for, or may even miss the ship entirely if you're firing from 2500+ meters away, and your error was of a great magnitude.
If you estimate a range that is too long, the torp will lead too much, and will either hit the prow, or again, miss. Those effects are even more pronounced when your fish is slow, and/or your target fast/small.
Granted, precise range accounts for way less in the "perfect" solution than precise speed and AOB, but it does have an effect on the spot you will hit.