While the responses were about which dive bag, I'll say that a dive bag is needed primarily because you need quite a bit of stuff to go diving: BCD, fins, mask, gloves, wetsuit, dive boots, etc. It's a lot easier if you can assemble most of that stuff in a single bag rather than scattered objects. It helps carry it and prevent you from forgetting things. With all of the rest of the stuff you need to remember when traveling it's a lot easier to have all of that assembled. It also helps if something is wet to keep it with the rest of the wet stuff or just in a known location. Further, it helps to keep all that stuff together when stored at home. You won't necessarily keep it all in that bag but again you can keep most of it and reduce the time to assemble stuff for the trip. As people mentioned above, there are a variety of ways to carry the stuff beyond an actual bag.