I disagree with this. The debugger is a very, very precise tool and it can be used to gain very, very precise insights into certain code, however, very often the debugger is just too precise. It is pretty much like trying to understand a large chip by looking at how gates flip and flop.

Of course, if the code is horrible enough, then you might need to switch down to actually tracing line by line and opcode by opcode, potentially even using a debugger, but for most sane code, I think it is possible and faster to understand larger blobs of code at once.