Vacuum breakers are a safety device required by some municipalities to hook your sinks into the public water supply. In a worse case scenario, it prevents chemicals in your bowl from being sucked back into the public water supply causing contamination. Some places require it and some places do not, so check your local building codes. A plumber can usually install a vacuum breaker even if it was not bought originally, but you will need to have your model number in order to check this.