A softbox could be used as a key light, and often is, but it’s classically more appropriate to use as a fill light. Soft light (which a softbox ideally creates

A softbox could be used as a key light, and often is, but it’s classically more appropriate to use as a fill light. Soft light (which a softbox ideally creates) is defined by diffuse shadows. Perfectly soft light would cast no visible shadows. An important creative job of the key light, which is called such because it’s the main source of illumination in a scene, is to create a sense of depth by casting shadows onto the 3D objects, giving our brain cues on how to translate them from 2D space into an accurate mental image. Usually harder lights (meaning those with sharper, more defined shadows) are used for the key to create this depth, then the fill light, well, fills in these shadows with additional light to walk them back, which is why soft lights are appropriate there. As with all creative rules, exceptions abound. Maybe you could make a virtual lighting portrait studio, have sharp white light on one part of the screen, then some sort of gray gradient on the other to fill in some shadow. Unfortunately, that would be most effective if they had a big curved screen and they were willing to sit very near to it :)