Why In amateur net terminology, "traffic" means formal written messages, radiograms, passed from station to station on behalf of third parties. A traffic net exists specifically to relay these messages, each with a standard preamble, address, text and signature so it can be copied exactly. The net control station asks whether stations have traffic to list, then pairs stations off to pass it.
Watch out The count of stations checking in is just the check-in list or net roster, not traffic; that bookkeeping goes in the net control's log, not into a message form.
Traffic = messages moving, like mail on a highway. "Any traffic for the net?" means "any messages to send?"