C is a system programming language which is old but with “bad” fame: undefined behavior, notorious memory related bugs, etc. Especially with Go and Rust having gone viral right now, C seems already forgotten by people. Nevertheless, IMHO, C is still a thing which is worth for you spending some time on it.
Whether you are are a C novice or a C veteran，I highly recommend you read Modern C if you haven’t read it before. Then you will find C also evolves stealthily and is not as primitive as you think. E.g., C11 has defined standard thread APIs like C++ has done, which makes C more like a “modern” language, not an outdated thing. You may get a new perspective of C from this book.
Regardless if you are a systems language programmer, DevOps, performance engineer or wear other hats, the more you know about the Operating System, the more you can do your job better. Take all prevailing Unix-like Operating Systems as an example, from kernel to command line tools, they are almost implemented in C. To study related source code can make you delve into Operating System internal deeper. E.g., I knew there is a taskset command which can bind specified process of a dedicated CPU, but I wanted to know the magic behind it, so I went through its code. Then I learned 2 things:
a) There is a “/proc/%pid/task” folder which records thread information of process;
b) taskset actually calls sched_setaffinity and sched_getaffinity APIs to attain its goal.