"C programmers think memory management is too important to be left to the computer. Lisp programmers think memory management is too important to be left to the user." Ellis and Stroustrup, The Annotated C++ Reference Manual.
In the mean time, in the messy world of writing software today, one does frequently enough come across the need for new safety primitives. Things that are provably correct but which the type system of your language does not support. In these instances, unsafe lets you lower down into systems code to build and safely wrap these new components.
But there exist safe systems programming languages. Safe systems were done in these languages. Just nobody cared, so they died or have no market share.
Mesa/Cedar: LOOPHOLE inside an UNSAFE module
Smalltalk 80: You can access the built-in VM instructions directly using the <primitive: N> syntax. E.g. basicAt:put: at <primitive: 61> avoids bounds checks and type checks.
Common Lisp: (safety 0)
Ada: Unchecked_Conversion, Unchecked_Deallocation
C#/M#: unsafe class MyClass<T> where T : unmanaged
Oberon: the SYSTEM module
Rust isn't any different, and unsafe exists in Rust for the same reason it exists in all these other languages. You use it to create new constructs the language authors didn't foresee. Concurrent Pascal is the singular exception: You can't do anything the language doesn't provide out of the box. Need something Brinch Hansen didn't think of? Sucks to be you.
If you want the rusty version of Concurrent Pascal though, you can have that. Put #![forbid(unsafe_code)] in your crate's lib.rs, or your Cargo.toml. Done.
There's a lot you can do in Ada without resorting to them, and even with using them it can be perfectly fine, such as (eg) "view conversion" of a register or memory-mapped location -- remember that a lot of APIs (and ABIs) have been kneecapped by catering to C's inabilities, so even if the idea is directly expressible in some higher level language it will be exposed at the lower level.
I'm not trying to fight some religious war here. You want to use Ada? Great! But don't pretend that the existence of the unsafe keyword somehow makes Rust unacceptably impure. The same escape hatches exist in every other "safe" language outside of the one toy example whose entire niche purpose was to avoid unsafe, and which no one uses now because the language is ossified and non-extensible.