It is used in the process of one of the secondary ways to radar identify an aircraft. Also, when ATC deems that the secondary is not necessary upon the aircraft getting within 15 nm from destination airport, they may instruct to turn off the transponder. Also during certain phenomena such as ring around, ATC may instruct the pilot to turn off the transponder to reduce ring around or if there is a high population of actual targets (aircraft) to reduce scope clutter.
100% WRONG, in 30 years ive never ask to have a TXP turned off because i didnt think i needed the secondary target.do you even know what happens to an ARTS/STARS data block when the TXP is turned off ?...IT DROPS OFF AFTER 3 SWEEPS WITH WITH OUT A TXP SIGINAL
You've never flown to Oshkosh or Sun N' Fun, Have you?