While I would say that the particular use for down-b that you're talking about is very niche, the general sentiment of using down-b from ledge makes for a huge array of useful options. Essentially, you have two jumps from ledge. And if you refresh your invincibility correctly, you can get more vertical reach from ledge than most other characters while invincible. And you also have the flexibility to attack out of it much faster than your typical ledge jump (Meaning pressing jump while holding onto ledge).
Down-b can also make for incredibly quick wall jumps from ledge, like Foo said. You can jump out of this and bair, which is, imo, her safest and most effective ledge attack. You can also do quick, slick perfect platform wavelands and instantly take center stage from ledge on stages like Yoshi's Story, WarioWare, and even Battlefield. If you simply down-b jump, you have the height and invincibility in order to waveland and take center stage on Dreamland, as well. With her down-b, your opponent has to defend both the corner ground as well as the platform above it. You always have two entry points on every stage, so your opponent can't effectively choke you out of the stage like most characters. Not to mention that if they get too close, you can down-b into them and flipstool off of their head, pushing them into the corner and taking center stage.
Down-b is the backbone of ZSS's movement game from ledge. Keep testing out stuff with it. She has so many options with it.
Bonus fact: On Skyworld, you can down-b away and double jump perfect waveland off of the side platforms in either direction because of their strange positioning. I'm beginning to think that Skyworld may be ZSS's best friend for recovery because the two options your opponent has to cover are so far away from one another.