> have often been confused as to why if you're writing an assembly program, you have to use at least a little C if you want to make Windows system calls

This isn’t true. The Win32 API is C-based. You can call a C-based API from hand-written assembly just fine. You just have to understand the ABI calling convention so you know how to do it. You can also write some C code to call the API and then get the C compiler to output assembly and then you can copy/paste the relevant portion into your hand-written assembly

> The classic example being Chris Sawyer writing nearly all of Rollercoaster Tycoon in x86 assembly but requiring just enough C for the system calls.

I don’t think he had to do it that way. Maybe he just decided that was the path of least resistance for him.

Ultimately, he has to have assembly call C at some point, since his own assembly code has to call his C function which calls the Win32 API (technically not system calls, as other commenters have pointed out). Maybe, given the Win32 API involves some rather complex data structures, numerous constant definitions, various helper macros, etc, he may have just found it easier to use some of that stuff from C instead of trying to replicate the complexity in hand-written assembly, or having to translate all the C header definitions he needed into corresponding assembler macros