Is learning C really a must for a programmer?

Many have this opinion. Somehow, by doing manual memory management allocation and garbage mangement one would become a better programmer.

I have little experience in C mainly because I can write terser and more readable code in Racket or Haskell. However I'm willing to give C a try just to learn these lessons that everybody keeps talking.

But I have a sneaking suspicion that I already know most of what is there to know. Can you list some useful lessons that a typical high level code dweller would be oblivious of?

C is like a portable assembly language. The point of it is to be very near to the hardware, without having to do manual register allocation. If you were to write a kernel for example, you would want to keep the cycle count low and the (non-hardware related) interrupts minimal, hence Haskell would probably be a bad choice; but C would be great for it.
Manual memory management doesn't make you a better coder though, it's just useful for a lot of things.