The bubble extractor will purge air from a line, tank or filter cartridge so your pump will not lose its prime.
Equipment like air conditioning cooling running on raw water will often pick up bubbles entrained under the hull that collect in or are sucked into raw water intakes. The next time you try to run the A/C it overheats because the pump has lost its prime. This is particularly troublesome in the case of water makers where any air will prevent the high pressure pump working.
The bubble extractor is located at a high point in the lines or on top of a filter so any air will be removed. It vents air but blocks liquid. To do this, it needs a positive pressure to atmospheric so the high point from which the bubbles are extracted needs to be below the water line or on the outlet side of a pump so the liquid level can reach the extractor and remove all bubbles.