It helps to make informed decisions, for example regarding memory locality, and it helps when they don’t perform as expected or required. It’s also difficult to get a good understanding of the properties of algorithms and data structures if you’ve never looked at how they are implemented under the hood. Learning how algorithms and data structures are implemented is probably the best way to learn the general principles.

And then there’s also some algorithms, like for example topological sorting, that are typically implemented as part of application-specific code, not by use of an existing stock implementation. When you know nothing about implementations, you might not notice that the code you’re writing happens to actually be an instance of a well-known algorithm or data structure.