And the point of the comment you're replying to is to ask what is "what it is supposed to do". How do you know what the answer to that question is, without documentation or a specification? And if you try to rely on just verbal communication, in a group of people probably larger than about 1, they're going to have different ideas about what the software is supposed to do.

Some of the most challenging problems I've encountered have been looking at code that does something. What it does is clear enough from the code. But why it does it, or should it do that, that is much harder to answer, particularly if the person who wrote it has left the company or it's been >6 months and they just don't remember.