when an area on a mechanical part is rough it may be machined smooth to help increase surface area contact between parts. this increase in surface area may help increase friction between faces of parts like fasteners.
for example, the inside face of bolts have a greater likely hood of holding tight against the face of the part and make it less likely the bolt will come loose.
the dimensions of spot faces are determined by machinists. designers might only indicate the presence of spot faces on mechanical drawings.
spot faces should be designated on a drawing by a dimmension and the the letters "SFACE".
