It's not necessarily a matter of being good. Writing efficient code doesn't mean that you can build a mental model for a reasonably complex projects, which will

It's not necessarily a matter of being good. Writing efficient code doesn't mean that you can build a mental model for a reasonably complex projects, which will naturally accumulate dependencies. If you're lucky, you'll get a mental model of the component you're working on, and some of the more problematic dependencies. That's where being able to dive into arbitrary code during execution becomes crucial. Especially if the problem occurs in a piece of code you don't have access to at the layer you operate, and hence can't display the value for easily, or at all.