Harry Ferguson "invented" the 3 point hitch. Part of the "value" in his invention was the ability of a smaller, lighter, lower horsepower tractor to do the work of something bigger.
Back then, almost every tractor spent a fair amount of time with a plow mounted on it. The load (or resistance) a plow exerts on a tractor is called "draft". It's effected by the depth at which the plow is operated. Draft control is the tractors ability to sense increasing or DECREASING draft loads as the tractor moves along. It moves the plow up or down (ever-so-slight sometimes) to regulate that draft load. As the plow is lifted slightly in tough going, it pulls down on the rear wheels, increasing traction through the tough plowing. After the tractor passes through the tough spot, and the draft load lessens, the plow will return to it's pre-set depth.
Draft control is of very limited use in anything but tillage. (i.e. moldboard plow, chisel plow, field cultivator, ect....)