It takes time to "context up" on code you haven't read in a while (or ever). You have to read it, understand how the parts fit together, understand any gotchas

It takes time to "context up" on code you haven't read in a while (or ever). You have to read it, understand how the parts fit together, understand any gotchas and what purpose it serves for other modules. All this takes time. Comments are just a way of speeding up the process of "contexting up". Typically it's done by saving a bit of that mental context as a comment. Ideally you want to save a piece of context that's easy to express, but took a long time to understand in the first place.