Fennel combines the simplest general programming language (Lua) with the simplest syntax (homoiconicity). The result is like a Lisp without all the esoterica.
Embedding Fennel in Rust allows you to ship user-facing macros written in Fennel without users having to setup a Lua environment beforehand (e.g. [1]).
[1]: https://git.sr.ht/~ioiojo/kiwi/tree/master/item/tests/data/c...