A rather large proportion of time in woodworking seems to be spent making decidedly irregular pieces of wood square. At least square in the sense that faces are parallel to one another, and perpendicular to edges. The operations then proceed from this reference form into any manner of angle and curve, but the starting point by necessity is the “4 square” form. The machine responsible for the reference surface and reference edge is the jointer. The planer addresses the surface opposite the reference surface. The table saw commonly addresses the edge opposite the reference edge.
Given the relative importance of the jointer in establishing the first two reference planes, one would surmise that any jointer is designed to achieve both flatness and perpendicularity. As with many things, reality is inconsistent with the theoretical. As a feature, the fence on many jointers are adjustable. Akin to an adjustable sole plate on circular saws and jig saws. There are conceivably instances where cutting or jointing at angles other than 90° is desirable. This would be a great addition in functionality were the primary function of cutting at 90° not compromised in doing so.