Why does the CDL manual instruct tank vehicle drivers to brake far in advance of a stop and increase their following distance?

49 CFR 383.119(b) (proper braking procedures for the motor vehicle when empty, full, and partially full); AAMVA CDL Manual Section 8.3.2, p.8-8-2

A partly full tank is unlike a dry van load; the cargo itself keeps moving and pushing on the vehicle as you slow down. Starting to brake earlier and leaving more room ahead gives you time to slow gradually instead of abruptly, which keeps the surge from building into something you can't control.