We've all been there. We go to our favorite Best Buy store, purchase the latest in Home Theater, then drive to the house, anticipating some serious surround-sound loud enough to drive the wife nuts (that's the whole point, right?). And then we realize the heartbeat of our system, the sub woofer, doesn't come with the cable necessary to make it go "BOOM!". Depending on which type of output/input you are dealing with, some receivers have an output with separate left/right, OR a single output, OR two outputs which allow for two subs. And then there's the other side of the equation: some subs have one input, OR two (yeah, a left AND a right, though with a sub, channel separation doesn't make any difference audibly). Therefore, it makes sense to purchase several of these "Y" adapters with a variety of ends, so you are never without. This way, you can drive home with a smile, confident you are not going to bang your head against the proverbial wall for coming up short, and having to drive BACK to Best Buy for a small, yet vitally-important cable connection.
Yes Sir... It's better to have it and not need it, than to need it and kick yourself for not having it (especially since you were walking all around it while shopping). Even if it isn't necessary for your current system, eventually the time will come when it will be. Trust me!
Just be sure to place it where it can be easily found when you DO need it.