Swap is used when the kernel runs out of memory and need to swap less used information to the disc to make space for information currently in use. In a good setup the swap will never be used but if it's needed and not existing it will make processes crash.
I have just started out testing BitTorrent Sync (btsync) on my Odroid-C1 and noticed that all the memory was allocated more or less straight away after starting the btsync service. So just to be safe during my test, to avoid crashes, I added a 8Gb USB stick to use for swap space. You can of course use the SD-card but that will take up space needed for the system as well as shorten the life of your SD-card.