It'll definitely show you the changes, but how far back do you start from? and how many files do you do this for? In addition, it doesn't show you how the dataflow changed; you need to model that in your head still.

Tests would also do this for you, but without the mental burden of brain compiling. It's nice to be able to set some breakpoints in code, then start a quick debugging session with the relevant test and see the data flow through the code - you can understand any function usually within 5 minutes.