Generic voices are a necessity too. They provide contrast to good voices and are great at adding without taking away from a point or scene. I.E. generic guy says “commander we have a problem” Commander then gets to say his cool line with awesome tone. Or generic background talk that you don’t want people to focus on. Like people chatting in a restaurant you want to be homogeneous so that Morgan Freeman’s voice is what you focus on.

Plenty of good reasons to want basic voice actors, though most anyone can also fill that roll, so probably hard to be a professional generic voice lol