It is called "oiling the wheels".
I would imagine the staff are not too happy about the move, the disruption, moving desks and cupboards, moving phones and computers, sorting out new seating arrangements.
If the staff are upset then they take days off "sick", their workrate drops, they all stand around in little groups moaning about the move.
The lunch cost far less than having a load of disgruntled employees with a low workrate.
It may be that the lunch came out of the budget for the move anyway.