think of the communication between server and client as a telephone conversation. on one end you have caller 1, who speaks into the phone, sending data through wires to caller 2. once caller 2 receives the information, he/she processes the information and responds, by sending information back through the wires.
when you're on the phone with someone, you can't reach through and hand the caller something - you'll need to send it through the wires, as always.