interface--practically every major language has a way to interface with code written in C/C++ or it interfaces with C/C++ libraries in some way.
speed--write optimised / number-crunching code (that can still be accessed from your other code written in another language).
computer architecture--learn about getting close to the "bare metal"... pointers, registers, cache, and the "volatile" keyword; learn about instruction set architecture (ISA), advanced vector extensions (AVX & AVX2), #pragma simd, prefetching, memory alignment. know the purpose of certain instruction sets developed for certain architectures (AVX-512 for Intel Knight's Landing, CUDA code optimised for Nvidia GPUs). or at least learn enough so that you can appreciate some of the points raised in Ulrich Dripper's article "What every programmer should know about memory". not many languages out there give you a chance to get that crazy of level of control, or allow you to build an appreciation for some of these things that are happening "under the hood".
basics can be learned very quickly--the basics of C can be picked up over a weekend. better still, what if the bottleneck in your code is just one or two loops, which could easily be re-written in an optimised form with just a few lines of C? see point #1 and point #2.
embedded systems / IoT / Arduino, etc. C is a very popular language for these kinds of systems. also, having a small footprint helps.
historical--many things in C/C++ influenced other languages. learn a bit of the history of how various concepts in programming languages evolved. have a bit of appreciation for why it has survived for so long.
operating system--learn about (or be able to access) Linux system calls (and perhaps other operating systems as well, besides Linux) and strace (track system calls made by an executable). here is a cheatsheet of Linux system calls: digilife.be/quickreferences/qrc/linux%20system%20… and going back to point #1, if your programming language of choice lacks access to a certain system call that you need, it might be possible to access it through C.