[xoxoxoBruce]

No no, people aren't standing in the street waiting for the building to be finished so they'll have some place to work/live.
If it's a company putting up a building for their firm, they always build bigger than they need hoping the business will grow and require more people.
The rest of the buildings are built by real estate speculators to rent space out. 99% of all cities have a surplus of unrented space. You know, vacancies, not built to demand, but in hopes of future demand.
If there is future business expansion, they might even have to build a bridge to accommodate those increases. That would be facilitating those increases not creating them.