Knowing the properties of a data structure and how it works under the hood is incredibly valuable when deciding if it's a good answer to the problem at hand. If you don't know why hashing is important or what it gets you, you might not understand why a hashmap is a very powerful tool. You might choose some other structure or hand-roll something involving manually managing multiple arrays, and down the line run into what feel like intractable performance problems.

That's just one example, pulled from people I've worked with. An understanding of fundamentals is required to do a real evaluation for yourself.