Is it necessarry? Absolutely not. Most people will tell you that it provides better leverage and heelside response. Personally, I've tried them both ways, (aligned and non-aligned) and I prefer them aligned. My reasoning has nothing to do with leverage and everything to do with comfort. I find the bindings to sit more comfortably on my boot when the highbacks are aligned with the heel edge.
Pretty much feel the same, plus it seems to allows a bit more lateral movement without being uncomfortable.