question about `compilation-next-error-function'

[Drew Adams]

Why does `compilation-next-error' add a new marker each time you call it?
For example, if you use RET multiple times on the same *grep* hit, then
multiple markers are created in the source-file buffer at the same hit
location - you get a new marker each time you visit a hit.

I'm not suggesting this is bad or should be changed; I'm asking why this is
the behavior, so I can understand better. Thx.