I'm confused about the example for trait_variant:

``` #[trait_variant::make(HttpService: Send)] ... ```

In the given example, they create a trait with an async fn that returns HtmlBody, then the trait_variant macro makes a duplicate version of the trait but additionally with a Send bound on the future. Makes sense, because users commonly need the future to be Send.

But... what's the point of having two traits? Could there not just be one trait, with the Send bound? Doesn't the Send bound only increase the places it can be used? Are there scenarios where you need the future to not be Send?