Ultimately it just feels better, over Keynote I’d say better alignment controls, less defaults to unset, collaboration and sharing which keynote lacks, zoom out to see the whole thing and move things about easier.
Also because it’s already an excellent design tool you never hit the wall where a diagram can’t be created in it like you hit often in Keynote/PPT/Google Slides
(I haven’t tried Figma Slides yet, this is normal Figma)
I'm not a Figma user and have no clue, and am wondering if it's worth giving a shot as a new user.