I place a .envrc at the root of our monorepo with a handful of env vars that configure stuff like PATH (to point to the various scripts, tools, and executables

I place a .envrc at the root of our monorepo with a handful of env vars that configure stuff like PATH (to point to the various scripts, tools, and executables that are used in the dev environment), override the sane default config files with an environment variable (extremely useful when chasing down specific bugs) and to redirect or control logging and debug info when running tools and systems. It makes it a breeze to pull in a commit and set up a development harness that pokes at whatever thing I need to poke at in my local environment. And it does it without changing a line of code or command line invocation, which is a big deal in polyglot environments with various build constraints (not passing -DFLAG=thing is enormous in a big C/C++ code base, for example). Even just being able to point whether a service is looking at a dev/production/local service/database is a big deal if you've invested in IaC and don't want to mess with any config files to do your work (as .envrc is probably in your .gitignore).