Roughness is easy to specify and easy to assume. Where a drawing names a value — Ra 0.2 µm, for example — it has to be measured, because two parts can look identical and measure differently.
Surface roughness where the drawing specifies it, typically on sealing faces, sliding surfaces, optical seats and any face where friction or sealing performance depends on finish.
Roughness is measured with a surface measuring instrument against the specified parameter, on the face the drawing names — not inferred from how the cut looked.
Finish drives sealing, friction and wear, and in clean applications it also drives how easily a surface releases particles. An unmeasured finish is a risk carried silently into the assembly.