D-throw is her second worst throw. It allows people to choose which way they pop up, and at low percents all the throws pop them into a relatively similar airspace.
Like take sheik vs pit, if she dthrows pit, he should always DI infront of her to stuff out any followup with his fair, but if you back throw him, you give yourself a guaranteed punish since his only means to contest you is the super slow bair.
And vs characters like Kirby who have giant brick aerials like his bair, it's so counter productive to not f throw. While characters like Marth who are broken and have a BS brain dead spacing tool like fair, f-throw and d-throw put him in a perfect fairings position.
Obv there are some characters that just suck so your choice of throw is irrelevant but yeah, you generally want to throw them in the way that reduces their options.