This will expose any weaknesses in the plan and ensure the primary site has been properly repaired before moving back.
Moving critical assets first may induce a second disaster if the primary site has not been repaired properly.
The first group to go back would test items such as connectivity, HVAC, power, water, improper procedures, and/or steps that has been overlooked or not done properly. By moving these first, and fixing any problems identified, the critical operations of the company are not negatively affected.
Source: HARRIS, Shon, All-In-One CISSP Certification Exam Guide, McGraw-Hill/Osborne, 2002, chapter 9: Disaster Recovery and Business continuity (page 621).