From
aviation.stackexchange.com:
The purpose of squawk code 2000 is to prevent aircraft entering a Secondary Surveillance Radar (SSR) area from transmitting a code that is the same as a discrete code assigned by ATC to an individual aircraft.
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Another use of code 2000 is on the airport: before power up you can select code 2000 to avoid code conflicts that could occur when you would maintain your code from a previous flight. It is standard practice in many airlines to select code 2000 after arrival at the gate.
Selecting code 2000 before shutdown prevents the aircraft squawking a conflicting code when it the transponder is turned on again. Squawk codes are in short supply so they get reused pretty quickly.