Specifically, Rust's _ is not a variable, it is a pattern that matches anything and so let _ = isn't an assignment it's specifically the explicit choice not to assign this value. If we wrote a "dummy" variable the compiler is forbidden from dropping the value early, that "dummy" is alive until it leaves scope, whereas if we never assigned the value it's dropped immediately.

In modern Rust you don't need the let here because you're allowed to do the pattern match anywhere, and as I said _ is simply a pattern that matches anything. So we could omit the let keyword, but people don't.