The primary purpose for vacuum breakers is to minimize the risk of damage from water hammer.
This is especially true when pumps are installed above an open source and discharging through pipe to a higher level. When the pump is stopped, the water in the discharge pipe may drain back to the source and cause contamination if a chemical was injected in the discharge pipe.
Even with a check valve in the discharge pipe, the check may leak and the water in the pipe can slowly drain out. When this happens, the falling water level in the discharge pipes creates a partial vacuum which  becomes filled with water vapour.
When the pump is restarted, it quickly fills the empty pipe with water until the water meets the first obstacle in the pipe system. At that time, the rush of water is abruptly stopped and the power of the rushing water is converted to pressure energy causing a high spike in pressure calling water hammer.
This pressure spike can and often does cause damage to the system. A longer discharge pipe will have a larger volume of moving water and result in a higher pressure spike.
If air is allowed into the pipe, it will cushion the flow of water and slow it down more gradually as the pressure builds. Water hammer and damage will be avoided. Vacuum release accomplish this.
The basic design includes a float device which is lowered when the water in the pipe is drained away, air is then allowed into the pipe and damage is avoided.