This step has to be executed mainly due to the fact that GIS services cannot be transferred from ArcGIS Server 10 to ArcGIS 10.2 for Server. They have to be rebuilt from scratch requiring an accurate documentation of your existing GIS services.
Note: A detailed description of services and configuration that need to bee documented you can find in the ArcGIS Resources - Migration checklist.

Make an inventory of your GIS services

Document the service properties