I'm more curious why it's beneficial from a gameplay/strategic-options-for-players aspect.
I would assume they've done it to add complexity. The more factors there are, the more difficult it is to evaluate them and make a decision. And from what little I've seen, I assume that there are interactions between the choices made in the Tech Tree and those in the Culture Tree. That would greatly increase complexity, and make it much more difficult to work out the potential synergies or anergies. I don't see how you could get such interactions without having two trees. (No doubt someone will come up with a way!!)