This new standard defines interfaces for request handlers and middleware. These have enormous potential impact on the PHP ecosystem, as they provide standard mechanisms for writing HTTP-facing, server-side applications. Essentially, they pave the way for developers to create re-usable web components that will work in any application that works with PSR-15 middleware or request handlers!
I faced an interesting question recently with regards to middleware: What happens when we go from a convention-based to a contract-based approach when programming?
Contract-based approaches use interfaces. I think you can see where this is going.