Having lived in bash for an awfully long time before coming to fish,
my fingers had learned to use a subset of the history substitution.
Most notably, when I needed to tell a program to [make me a sandwich],
I'd sudo !!, and there were countless times I'd mkdir <something>; cd !$.
Coming to fish, this was hard to get over.
[make me a sandwich]: https://xkcd.com/149/
This was before abbr became part of fish,
and in the discussion around how abbr would be implemented there was a suggestion about using key bindings to experiment with it.
From that discussion, I took the inspiration for these bindings.
The really nice thing about these versus abbr is that they expand immediately (no whitespace needed)
and they expand anywhere, not just at the beginning of the line.
For instance:
> which myscript.sh
/usr/local/opt/bin/myscript.sh
> less (!!)
works - and as a bonus over the bash use case, the !! expands in place, so you can edit it.