> what are the real benefits and downsides of this sexism (or of the ideal sex-fairness) for our society, besides the "social justice"?
Aside from the obvious, the bias, and people's knowledge of the bias, creates a perception that women are only there due to an unfair bias in their favor. This is toxic, and it damages the cause of reducing such biases, because it brings conversations back to gender equality when they should be about individual merit.
And that's what this is about: Individuals. Individuals with their own merits and qualities, not "individuals" as proxies for whatever group they've been assigned to. Fighting bias is all about making everyone see everyone else as just an individual, just someone just like them in most respects, not as some instance of a class to be treated identically to all other instances of that class.
Going for numerical equality is metric-based thinking, and it relegates individuals to being instances of a class. It's like saying "Oh, you'll have so much in common! You're both women, after all!"
(Metric-based thinking: Optimize for what's easy to measure.)
> I mean, if this practice is not beneficial to anyone (except the girls, [EDIT:and also perhaps the utterly smart guys in the world as the valuation of this asset for them increases due to this hiring preference]), they probably would just drop it in the end...
Not if it becomes ingrained, or if it has benefits to the organization which the leadership thinks outweigh the detriments to the workforce.