Why is it trivial to take a unfamiliar code base and dive into it, hunting down calls, working through the stack to understand what / how a particular function

Why is it trivial to take a unfamiliar code base and dive into it, hunting down calls, working through the stack to understand what / how a particular function works. This is a hugely important part of familiarising oneself with a codebase - it leads to many useful discussions. Who should you talk to if there is problem in a function halfway down. If I want to add a parameter to the top function, what might have to change down the stack? I am not sure why thats trivial