It'll add money to the cable manufacturer's bank accounts!
In large part, it's there because the market expects it.
There are two different things (well, more, really) that you can do with 2 sets of binding posts: bi-wiring (most likely no benefit whatsoever, see
this link for a good, in depth explanation. Incidentally, that's what I did with my speaker cable, although I only did it for fun, not because I expected any sort of real benefit.
Bi-amping (as seen in the article above) is using two separate amplifiers to drive different portions of the speaker. It is slightly less sketchy in concept than biwiring, but you are still only likely to see benefits with an external crossover between the amps rather than the internal crossovers between the drivers.
