Basic debugging knowledge is helpful, but I've found the ability to think about distributed, networked systems all the way down at the system level to be extremely helpful. We all encounter systems that are ill-documented, or complete black boxes, but peering at them with tools like `strace` / `jstack` (when a service is doing something itself with data) and `tcpdump` (when a service is asking another service to do something) opens up all sorts of possibilities.

Yes, distributed tracing tools do a much better job of this, but it's not always a given that a service will be instrumented. The generic tools (almost) always work.