First, adding the the Customer Location field in the project header provides consistency to functionality available throughout the software. In addition, it could "fill in" the project site location field. By changing the location in the header, the client knows specifically which site this represents for that customer. In addition, the customer's site locations would then already be defined when using the Service Management module and prevent double entry of the same data.
NOTE: It may be of value to add an "override" flag next to the project's site location field in the event that this address would need to be different.