On businesses: employee pay goes up, potentially pulling people from other lower-paying jobs [this is why your plantation workers finally showed up]. If it's a service building, service quality for the customer (i.e. that kind of happiness) goes up. If it's a production building, production/productivity goes up. Job quality goes up.

On residences: quality goes up (i.e. housing happiness). Unless you made the mistake of promising better housing in an election speech, there's usually not much value in increasing the housing budget. You get more happiness (and more income) from building new housing for the homeless, and/or better housing as the eras go by.

Thanks a lot!