Sometimes producers make one tune that is totally different to their usual output, so a label that would sign that sort of tune ain't gonna commit to signing the artist if there aint gonna be anymore tunes like that.
And many smaller labels don't want the hassle or don't have the resources to keep an artist paid (i.e. getting them regular dj gigs....etc)
Being signed usually means at least 2 or more album deals and exclusive signings mean they can only release on said label.
Getting a track signed is one contract for that tune or tunes and the artist can continue to get other tracks signed in conjunction.
They exist because nowadays any man and his dog can release anything, historically band/people could only get stuff released by bigger labels so a deal would revolve around the artist as a whole. That's my understanding anways
Well I lease a couple of my tracks out to big Jeff at Gautamano bay says couple hours of water boarding and my music gets even the ardest jihad talking. Anyway yeah what he said leasing a track is different though for instance I might lease a track to a video game or a tv advertisement. So there's that aswell