I came across lod90 images in this default set for cars by Slig. Each replacement texture had an associated lod90. From the text, I get the impression that they have something to do with the traffic that appears in hood view, but no more than that.

So, what do they do, how do they work, and (most importantly) do I need to make them if and when I do my own car defaults?

In general, lod90 textures are used for the neighborhood view. I don't know for sure since I've never made a CC car but I don't see why they wouldn't be the same as other objects. If my Sim has a red car in their driveway I'd expect to see a red car from the neighborhood view. Personally, I'd include them just to make sure everything works the way it should.

Thanks. I'm still trying to decide on what'll be in the final replacement sets, but I will do the lod90 textures as well.