transponder
The reason one switches to standby before changing the code, is that if one cycles the code numbers it does flash other transponder codes.
Which is also a reason for having "Ident".
The transponder is a radar aid, it is not a training line to air traffic control.
If there is no transponder, or its off , ATC just read a target: no identity, no altitude no TAS/IAS.no supplementary information