That way, if someone is named "Billy" or "Alex", you won't be spending years mistakenly assuming they were a man while they could have been a woman.
This also has the beneficial side effect of covering trans individuals who use a birth name while having another gender identity.
Even more useful for usernames such as yours, "dionian". I do not know if it is your first name, last name or made-up username. So I have no clue what your gender is and I'd default to "they". If I knew, I could call you by the right pronouns.
It's just a slow shift away from assuming everyone on the internet is a man.