The idea is that membership of boards is often chosen by existing members of said boards, and that people (not always, but on average) have a tendency to choose

The idea is that membership of boards is often chosen by existing members of said boards, and that people (not always, but on average) have a tendency to choose people who are like themselves. And boards are typically male-dominated due to historical sexism, so the goal is to disrupt that inertia with an external push towards a more diverse group, with the hope that it will then be self-sustaining going forwards. And this is considered desirable because it means that opportunity and power is more evenly distributed across different sections of society.