Personally, I don't care for Atomic CSS, but it's good to understand why it appeals to others:
Atomic CSS offers a straightforward, obvious, and simple methodology. Classes are immutable - they don't change. This makes the application of CSS predictable and reliable as classes will always do exactly the same thing. The scope of adding or removing a utility class in an HTML file is unambiguous, giving you the confidence that you aren't breaking anything else. This can reduce cognitive load and mental-overhead.