Custom macros are very commonly exported by libraries, but it also has nothing to do with homoiconicity, Rust supports a fairly similar procedural macro model as well as a more higher level pattern matching one and isn't homoiconic.

A macro system that manipulates the syntax tree directly has little to do with homoiconicity.

The real advantage is simply that the syntax is very simple and consistent but there is no need for it to be homoiconic to be that.