> code is already formal enough that you don't need a formal visual language

Code can be formal enough but in reality it is almost never formal enough. Many constraints can be expressed imperatively (eg. someone validates the number of related objects in the create view) and in unexpected places (across repositories and libraries and maybe not even in code but in some stored procedure). And then it is all in flux and can go away the moment you change your stack.

I don't know if UML is perfectly flexible (maybe?) but its sure benefit is a source of truth that does not depend on your implementation.