Quoting from the rust book:

> Unsafe Rust exists because, by nature, static analysis is conservative. When the compiler is trying to determine if code upholds the guarantees or not, it’s better for it to reject some programs that are valid than accept some programs that are invalid. That inevitably means there are some times when your code might be okay, but Rust thinks it’s not! In these cases, you can use unsafe code to tell the compiler, “trust me, I know what I’m doing.” The downside is that you’re on your own; if you get unsafe code wrong, problems due to memory unsafety, like null pointer dereferencing, can occur.

> There’s another reason Rust has an unsafe alter ego: the underlying hardware of computers is inherently not safe. If Rust didn’t let you do unsafe operations, there would be some tasks that you simply could not do. Rust needs to allow you to do low-level systems programming like directly interacting with your operating system, or even writing your own operating system! That’s one of the goals of the language. Let’s see what you can do with unsafe Rust, and how to do it.

(I would also disagree with that statement, but in a different way: Rust enables zero-cost abstractions, but nothing inherently means they have to be. I can also make quite costly ones, and they'll compile.)