The vent allows air contained in the tank to escape from the top of the ballast tank and be replaced by water entering through the open Kingston valves situated at the bottom of the tank.
When on the surface a submarine's ballast tanks are filled with air which gives the vessel its buoyancy and in order for the submarine to submerge water is taken into the ballast tanks through the open Kingston valves in the bottom of the tanks, destroying this excess buoyancy. As the ballast tanks contain air when on the surface it is necessary to allow this air to escape, so that water may then enter the tanks, and this air is allowed to escape via the opened vents in the top of the ballast tanks.
The vents which are used to allow water to enter the submarine's main ballast tanks when it submerges are the main vents and it is air escaping though these that accounts for the spray sometimes seen when submarines dive.
When the submarine surfaces the main vents in the top of the tank are closed and high-pressure compressed air is 'blown' into the ballast tanks, forcing the water out through the open Kingston valves at the bottom of the tank. Once on the surface, in order to conserve supplies of compressed-air, a lower-pressure compressor is then used to remove the remainder of the water from the tanks.
As a side note, it is a common misconception that adding or removing water ballast in a subsurface vessel changes the vessel's buoyancy. In fact, it is the vessel's displacement which is adjusted to attain a positive, negative or neutral buoyancy. Removing air from the ballast tanks increases the vessel's total water displacement, and when that displacement is greater than the vessel's weight, the vessel will rise to the surface.