You must be kidding - are you? I guess your question is as sensible as asking why is it good to have men on the board, specifically? Must be some reason given that there are that many - oh right, its men picking others like them.
And this is the problem - nothing special about women, but in case you haven't noticed yet - there are differences (beyond the genitalia). Different approaches to problem solving, different key strengths. And this is the key - higher diversity on a company board makes the company more successful. Different view points help to better analyse and solve problems, rather than having everybody looking from the same angle and coming up with the same old remedies.
https://www.2020wob.com/learn/why-ge...ersity-matters
http://www.catalyst.org/media/compan...cording-latest