It's a form of defense in depth. Ideally your application has specific test cases, property-based testing, linting, and whatever other forms of static and dynamic analysis you can think of. But if your code is obfuscated and/or you don't have a clear mental model of what it does, that adds a layer of uncertainty and could potentially hurt debugability.
Linked lists, hash functions, etc are mostly solved problems with clearly defined interfaces, built and tested for edge cases by humans. Each regex is a special snowflake.
