The custom stuff in our config was mostly the need to create standard styles for components. Eg. button large, button small, etc. to correspond with our design system.
For example, here's tailwind code required to create a button large: bg-violet-100 text-violet-700 text-base font-semibold px-6 py-2 rounded-lg
How the hell does that help with consistency? You have to copy paste that mess every time you create a new button, and then guess what happens if the button you copied from needed special margin? Try debugging that if something looks wrong. Also try turning that into dark mode.
Now you've got to search through the spaghetti to find what's custom about this button and fix it every time.
You know what's easier? Creating a ".btn-lg" style to fit our design system, and just typing .btn-lg each time. It also has the bonus of making your code actually readable.
Maybe this is solved with the groups functionality in the latest tailwind, but this is just one of the frustrations I eventually developed.