What is Transponder Code 2000 actually used for?
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.

If you are flying in the USA under Visual Flight Rules (VFR), you will be assigned (implicitly) code 1200. Upon entering a SSR area you might get assigned another code, but often you will keep code 1200 if you are not near class C airspace.

If you are flying Instrument Flight Rules (IFR) outside a SSR area (e.g. entry into the US from oceanic airspace), you will use code 2000. Upon radar contact you will then get assigned a discrete code. 

From FAA Advisory circular 91-70A - (Oceanic and International Operations):


  2.  EXPANDED OCEANIC CHECKLIST.
  
  .......
  
  g.
  After Oceanic Entry
  
  (1) Squawk 2000.  Thirty minutes after oceanic en try, crews should
  Squawk 2000, if  applicable. There may be regional differences  such
  as Squawking 2100 in Bermuda’s airspace or  maintaining last assigned
  Squawk in the West Atlantic Route System (WATRS). Crews  transiting
  Reykjavik’s airspace must  maintain last assigned Squawk.


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.