There are a number of reasons why you might want to achieve gender parity. There is a really strong argument for parity for parity's sake. We can't ignore the fact that 50% of our population is composed of women, and yet, they do not represent positions at organizations that reflect that percentage.
There are, as Alexandra pointed out, many benefits and outcomes that people talk about, the effects that more women in positions of power might have, especially when it comes to being on boards and in senior leadership positions. People believe that women make different decisions, that having more women on boards and in senior positions could lead to improved corporate performance or corporate responsibility.
The problem with trying to define exactly why or what kind of outcomes you might have for increasing gender parity in positions of power is that 10 years down the road, you end up with a program that may or may not achieve those outcomes, and the criticism then falls on the parity initiative.
You really have to be careful. Women didn't create these problems. But, of course, the parity initiative is going to be the one that will be blamed, right, if these outcomes are not achieved. It's great to have these ideas and expectations of outcomes but, at the same time, the strongest argument is the demographic one.