I realize this is a basic and naive question so I won't be offended by short responses, if any. What is the fundamental advantage of using a single speed frame over a frame from a geared bike?

I'm imagining using one cog and some spacers (using a converted frame), and I could swap out wheels with my road bike in a pinch which I'd count as a plus. Is it just the extra weight? Or some other reason to prefer the narrower dropout?

Is there an advantage having to do with the angle of the chain on the ss frame as opposed to using the inside ring on a geared bike frame? What else is there?

I'll just monitor the answers. Thanks.