Data migration is the moving of data from an existing application to a new application or a newer version of the same application.  This change is necessitated because the current application(s) is unable to meet new business needs. 

The challenge is the old application has some valuable data, often painstaking collected over time, that may not have a home in the new application AND the new application needs data that may not be in the old application hence not available.

The investment in the new application is only realized if the new application is fully functional and has the data in a format usable by the new application. To state the obvious - The new application is useless if it has unusable data in there.  

Data in the new application usually requires data to conform to the new business rules because the business has evolved since the old system was put into place.