It wasn't needed. It just happened. It's like saying why god would have it rain when water could just show up in the ground. We know it does rain, and we know with a high level of confidence that the Bang happened. Again, it's not a myth some people are trying to push; it's the theory based on all the physical evidence we've collected. In fact, the universe is still "banging" as we speak.
The point is that essentially we know the Bang happened, so there's no reason to make it seem like there's a choice of whether to believe in it or not. If you have no reason to disbelieve it (some religion telling you things didn't happen that way), I find it hard to justify challenging it too much. As I said because, the Bang is much less controversial than evolution. Denying it is like denying that the Earth moves around the sun.