The site was originally an HP campus, so the office space was already there; they replaced 80% of the former parking lots and office buildings with green space—

The site was originally an HP campus, so the office space was already there; they replaced 80% of the former parking lots and office buildings with green space—increasing CO2 absorption. The new building also allows them to consolidate employees from many of the 100+ other properties they lease within Cupertino and Sunnyvale. This further reduces the need for intercampus transportation. Hopefully these newly unused office buildings can be converted into residential space (edit: but Apple does not own them, so it's not their choice).