What’s the one thing we, as developers, do with source code more than anything else? We read it. Sure, we change it, and even occasionally we write new source code, but, by and large, we read source code. We read it to know how to use it. We read it to know where to change it. We read it to understand what it does.
But, it’s not just the content - the code itself - that affects readability. How it’s presented matters and if we’re going to talk about presentation, we have to talk about typography.