The common standard for the height of the center of a fire hydrants outlet is about 18 inches from the ground. The radius from that center point to the bottom of the outlet is 2.5 inches which is where body of the coupler will be, plus some room for the 3-4” handles attached to either side of the coupler body to be able to turn to thread the coupler onto the outlet. So that’s about 11” of clearance to the ground you’d need plus a few extra inches of some extra room added for your hands/arms or the fact that over time hydrants can kinda “sink” into the ground diminishing the clearance further.

If your local fire department uses a 4 way hydrant valve to connect to the hydrant for uninterruptible in-line boosting like this one

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Then that’s a bunch of extra clearance you’ll need.

And then there’s just the fact that the less snow there is, the easier it is for them to just clear away themselves.

8” isn’t an exact number as much as it is a ballpark for when snowfall around a hydrant goes from a minor pain in the ass to becoming a potential safety issue.