Most Programmers Can't Read Code

Unfortunately, when code is useful enough to survive for many years, the people who specified, designed and wrote the code are long gone by the time some unfortunate developer comes along and has to understand the code (e.g., to add features or fix bugs). So being able to figure out the original purpose of the code under such adverse circumstances can be a very useful skill to have.

In a startup there isn't a lot of legacy code, but in big companies that have been around for years or decades, old code like that is very common. (However, even in a startup, you might need to be able to make sense of poorly-documented open source code whose developers don't have the time to talk to you.)

It would be easier to re-write it if you had an exact spec of what it needed to do. But if you had to reverse-engineer the spec from the code itself (which is, in my experience, far more common when working with old code), re-writing it would be harder than reading it, since getting the spec would involve reading the code first anyway.

"It's that most programmers don't write code with the intention of having it read by other programmers."

I think that's a very important point. Nobody would ever try to read a novel written by someone who didn't care about the reader's experience, but we rarely see novels that bad because nobody would pay for one. However, code that's written without the reader in mind is unfortunately very common (it may, for example, be a quick solution for a short-term requirement that works well enough to be somewhat useful).