It just keeps it all in the portafilter as opposed to all over your kitchen, I get the same measured dose out of my grinder every time... Pull out the ring, tamp your coffee and away you go...
I don't have a dosing grinder, so my grind level is done by eye since my dope I mean coffee scales broke. You could make a ring like that to give you a better eyeball level. But then I'd still need scales to work out the dose and where to mark it, initially.
It just keeps it all in the portafilter as opposed to all over your kitchen, I get the same measured dose out of my grinder every time... Pull out the ring, tamp your coffee and away you go...