Because if you don't know C, you don't know what's actually happening when you write software.

Because if you don't know C, you've only seen the map, not the territory.

Because if you don't know C, you're looking at the finger, not the moon.

Because if you don't know C, everything you think about computers is a leaky abstraction.

The counterpoint is that flawed models are often good enough. Newton's theory of gravitation comes to mind.

But if you like knowing what's going on, either for personal satisfaction, or because what you're working on is not fully encapsulated by a different language (inevitably, originally, written in C), then C is both interesting and useful.