* Legacy code
* Embedded (including drivers, bare metal etc.)
* Portability, FFI, etc.
Performance used to be another major reason, but in $CURRENT_YEAR, if that's your goal, C++ and Rust are just as performant and there's very little reason not to opt into them.
The other major reason to learn C is that it's a great "hardcore introductory" language that will expose you to foundational concepts like pointers and memory management (it's really hard to understand what problems Rust is trying to solve if you're not familiar with malloc and free, for instance), but if you're already programming that's less of an issue.