You have text file that contains structures the editor knows about. You can edit and modify syntactically and semantically incorrect structures, leave them broken incomplete, etc.

Then there is a strict structure editor idea, where editing is writing ensures correct structures, modification happens from correct structure to another correct structure. That has never cached on. It turns out that editing code is more than just writing down the algorithm. It's also sketching, doodling and playing with ideas, ...

Lisp environments have the best of the both worlds. You have text, but also alive structures that live in the runtime image and can be saved. Close match between text syntax and the internal data structure makes it pleasure to work with.