You might be waiting (via select) for multiple replies; you don't know whether B will (say) grab something out of disk cache and chuck it back to you, or if it'

You might be waiting (via select) for multiple replies; you don't know whether B will (say) grab something out of disk cache and chuck it back to you, or if it'll fire off a seek command to the spinning rust and take a nap just as your network round-trip completes. Having two (or more) messages in (separate) pipes is the point of using select.