While Apple's Photo Stream is a great way of getting photos to sync across multiple devices, there's one glaring problem. Photo Stream doesn't support video, presumably because Apple doesn't want its servers full of hours and hours of 1080p videos showing cats sleeping.
It makes sense from a technical standpoint, but that doesn't help you if you need to get a video from one device to another. There's syncing with iTunes, and doing the iPhoto jig in the hope that everything works as planned, but it's a pain, and not always practical.
That's where PhotoSync comes in...