That's an awesome point I really wish I had made myself, so I'll just shamelessly rehash it to drive it home: Knowing how the system actually works down to a low level can help you avoid inadvertently writing code that will perform badly, even if you're dealing with code isn't otherwise required to perform above "reasonable" and thus doesn't call for low-level tuning.

Let's take database queries as an example: Knowing something about how the database is implemented will help you avoid writing queries that are pathologically slow, even if the queries you need to write aren't critical path enough to benefit from going to great length to tune them to the implementation.