A friend of mine wrote once a short story in which the main character (1st person narration) was never named, nor was their gender ever revealed. They were still important, and the reader still cared about them. It was actually a really cool experiment!
But yeah, I think the point is that it's better to care about and develop all characters. Creating characters is the closest the average person can come to actually creating people; there aren't undeveloped people in this world, so ideally, there shouldn't be undeveloped characters, either.
And, when I'm reading someone else's story, I like to piece together (and invent) information about so-called unimportant characters to make a richer and more enjoyable story and world.