If it is a pure function it makes the call depth irrelevant. While not always the case, it seems to be pretty common in Ruby development to keep things as pure as possible.
No, it's not irrelevant. If you're trying to figure out what gets passed up the call stack, you need to trace it through. Being able to plug in your own values into a single method call via the REPL isn't helpful if you're trying to analyze what really happens when it runs. Either you need to run it under a debugger, or manually trace it through. In a static language you just have to read the type signatures.
As mentioned by JackC, if you need to understand the context in which a method is called, use a REPL. But to understand what a pure method does should be context free, by definition.