Never used it personally, but I've seen them used.
It presses down on the valve, which releases most of the pressure on the retaining ring. With the tool you can remove or install a spear in about a minute. Without the tool it takes a bit of prying to get at the ring. I know people do just fine without the tool but, I'm pretty sure I'd be damaging those rings. I'd definitely be doing a bunch of cussing.