Very much so. I've spent a lot of time trying to decipher code and the biggest questions are usually "why is it doing this" and "what's this large piece of code

Very much so. I've spent a lot of time trying to decipher code and the biggest questions are usually "why is it doing this" and "what's this large piece of code indended to do". Reverse-engineering all that can be an utter nightmare even with tests. I suggest that people who disagree consider, for example, the word "reverse-engineer" as in how they might reverse-engineer a car. Usually you have this kind of problem when it's NOT producing desired results and the problem is how to make it do something that's needed without affecting some set of usecases that you don't understand yet.