For the same reason why we need quotas for businesses

Having a role model is hugely important for people picking up something.

If some girl plays chess and looks at the professional players and big tournaments there are no women there. So she likely will not pursue that path professionally. If there is a women's league then there the guarantee that there will be visibility for the winners which then creates more idols for young people and over the years increases the level of play until they are equal enough.

Not to mention the chess-clubs favouring boys on their training since they have a bigger chance to make it big and shine a spotlight on that club that produced this talent. With female only tournaments it's easier to create a name for yourself if you treat both genders equal and create the same talent for both sides there are so many fewer players.

Sure sounds dumb on paper but it's actually really necessary in order to create a pathway to more professional female players

I'm going to assume you aren't trolling.

Women traditionally have been discouraged from competitions, including chess. We are at the stage where we should be creating extra opportunities for women to be involved in these competitions. If we didn't, tradition and systemic practices would continue to discourage women.

Chess has no male category. There's co-ed, and female. This allows an extra space for women to compete against each other, feel safe, and make connections and friendships with other women in the minority. While still allowing them to compete in the coed category on a level playing field.

We will most likely continue to be at this stage for generations.

Having a women's category for chess is a way of creating a safe environment for everyone to thrive.

Only counter argument I can give you is have a bit of empathy.

Imagine that you were a young girl and saw this game. Chess. And you wanted to learn how to play.

The first person you asked was your mother and she told you oh that's a boys game. But you still wanted to learn.

So you decide to just read up on the rules. You don't really know anyone that plays. So you just kinda play vs yourself and redo. Old GM matches you found in old magazines. None of them are like you btw. They all seem like old men that live on the opposite side of the world.

You really like the game tho. And eventually you realize oh. There are actual clubs/tourneys near here I can go and play with actual people!

You go. And there's not a single girl there. Because of all the hurdles you had to jump. Most others that would have been interested just quit.

So you get there. You are obviously underprepared because you didn't have the support system that made learning it easy. Because of that you just get wrecked.

In addition to that you probably will have to hear some dumb jokes. That if you didn't have such a rough patch to get there in the first place they might have been funny to you. But they aren't. They just felt like you were being punched down.

Having exclusive tournaments and leagues is a very minor way of rectifying awful historical disparities. It's not perfect and it's not a solution by itself. But it's absolutely needed.

If the gender ratio was something like 1:4 or less, you'd have an argument that everyone should just get along. But there's like 16 times more men in chess than women and separate tournaments means the gals won't have to learn chest thumping to deal with the asshole faction they can simply be catty which they already know how to do.