Save for later would be awesome as an additional feature as all our brains have been conditioned to treat the favorite as a like or sign of approval.
The star serves as a positive feedback loop for the tooter as to what kind of content is working.
Also when someone likes your toot you are put in touch with a person that is on the same wavelength as you and you can build a connection from there.
So we are just abusing the fav button to friendly poke each other.
But there are some toots that have vast amounts of info, and a dedicated button to save for later would help bookmark the post.
(I know I’m being shameless asking for features, especially when I’m not the one who has to do the implementation.)