I've taught the labs for my university's Intro OS and Advanced OS courses, and I've heard this question many times. Here's the short answer: everything you do will eventually interact with the abstractions that your OS provides you; sometimes (often?), that abstraction leaks out implementation details. When you're confronted with an otherwise confusing problem (e.g. Why do I get shit performance when I read one byte at a time from disk?), understanding what the operating system is doing is going to be the quick way to figure it out.