Title explains.
Above is the reason. The question is, why not? It makes the interface feel more natural.
P.S. I find the progress bar under play buttons can do this, but it's not smooth, and can't position precisely.
P.P.S. Also hold "previous track" to reverse.
SPOILER
This feature happens in WMP, and other famous music players. It's useful to skip lead-in while previewing the song, and the user don't have to waste time in opening the editor or play it before determining whether the song is nice enough to be played or not.
Above is the reason. The question is, why not? It makes the interface feel more natural.
P.S. I find the progress bar under play buttons can do this, but it's not smooth, and can't position precisely.
P.P.S. Also hold "previous track" to reverse.