Associated types serve much the same role as generic parameters in the context of traits, but with the crucial distinction that associated types are specified by the person implementing the trait for their type, whereas generic parameters are specified by the person calling the methods defined by the trait.

In practice you could basically just have generic parameters and not bother with associated types, but it would be much less nice to use these APIs.

Here's an example of how an API would change if Rust didn't have associated types:

    let x = [1,2,3]; // an array of i32
    let mut y = x.iter(); // an iterator over it
    y.next(); // pull an item from the iterator

If Rust didn't have associated types, then that last line would have to look like this:

    y.next::<&i32>(); // if Rust didn't have associated types