Asynchronous results in the Play 2.0 platform (scala)

reading documentation. Claim Replay:

http://www.playframework.org/documentation/2.0/ScalaAsync

"Because of how Play 2.0 works, the action code should be as fast as possible (i.e. not block).

The following will show how to write an asynchronous result. Can someone broaden the question of why "action code should be as fast as possible"? How does switching to an asynchronous result affect how the request is processed at a lower level? What are the benefits besides abstraction?