Personally, I don’t find debuggers essential in my workflow, but I do think that they are a lot of fun and they can reduce stress when working with complicated code. Moreover, I don’t think that you can watch this video without having to admit that Gallium is going to be freakin amazing once it becomes fully mature.
I don’t think that I would say that Gallium is 1.0 blocking, I would just say that it will make working with Julia a lot more fun. Also, the possibility of being able to use it in a Jupyter notebook would be fairly mind blowing.
The point is that the prospect of a high quality debugger excites me quite a lot, it would make my life a lot easier and it could lower the barrier for newcomers to Julia. Also, I think that I should reiterate that the presence of a debugger makes Julia look a lot more modern to outsiders.
As for the original question, I think that it kind of answers itself. Debuggers are a tool for finding out exactly what caused your code to behave the way it did. Yes, you can use the stacktrace/printing thing that Chris explained, but a debugger is so much more elegant.