Perhaps this is obvious, but the transpinder gives a correlated radar target. Without a transpinder, ATC sees either a small dot or a plus on the radar scope, which are both difficult for the computer to track. Transponder-equipped a/c give a digitized beacon slash on the scope, which the computer can track accurately and correlate to the a/c position.

As far as data for a center datablock, first line is callsign, next is altitude info (current alt and assigne), third line is computer ID number (a unique number for ATC use) and ground speed in knots. Fourth line is programmable and could include a/c type, destination, misc info entered by atc.