In general, Future is a trait, which means if you want to return a Future, you have to either create a trait object, or use the nightly-only "-> impl Trait". Boxing is the most straightforward way of creating a trait object.
(I haven't dug into the specifics of this specific example; that's what I assumed when I read that line. Maybe the author also knew this rule of thumb and did it unnecessarily...)
(I haven't dug into the specifics of this specific example; that's what I assumed when I read that line. Maybe the author also knew this rule of thumb and did it unnecessarily...)