In most cases it's a request for more of the maintainer's time to review, fix, and merge a change which they themselves likely don't need (otherwise they would have added it). In my experience, at least 80% of the time I end up spending more time and effort on accepting someone's change than I would have writing it myself. It only becomes net-positive when a contributor has a long-lived interest in the project and turns out to be a capable maintainer with similar sensibilities; even then there is significant ramp-up time. 99% of the time this doesn't happen; most contributions are drive-bys.
P.S. It so happens that that we previously interacted on that very PR thread: https://github.com/BurntSushi/toml/pull/65#issuecomment-1919...