It finally clicked for me after reading the chapter [1] in "Functional Programming in C++" [2] on monads; partly because he gives lots of examples in C++ and reasons why they are useful. I've read a few other things too, so I'm not sure if it was the sum totality of all of them, but that book helped. That does require being familiar with C++, but that is probably the closest to C where monads can be clearly modelled and, as others have noted, it is easier to understand the concepts in a language where they can be more concisely modelled.

[1] https://livebook.manning.com/book/functional-programming-in-...

[2] https://www.manning.com/books/functional-programming-in-c-pl...