Spotify does have a general repeat mode, but the only way to repeat one song is to put it in a playlist all by itself and turn on the repeat mode. Nobody wants to make a one-song playlist. Sometimes we're in the middle of an album or our Starred selections, we get fixated one song, and we want to play it over and over with minimal effort.
It seems like a simple enough request for Spotify to make this a reality, although we suspect there's some legal issue at hand that's prevented this from happening. Regardless, the users want it, and that's enough of a reason to get it figured out.