"Given all the variations in powder and charges, how can they possibly be of any use?"
If you read the materials that come with the die set, you'll see that the included scoop generally throws in the mid to lower power range for a particular cartridge with a variety of powders.
That makes it useful for those powders that are listed in the Lee data sheet that comes with the die set.
When you get right down to it, many of the powders that are suitable for a particular cartridge have similar densities and similar charge weight ranges, and thus are listed in the data sheet for use with that scoop in that particular cartridge.
Those powders that fall outside of a useful charge with that particular scoop in that particular cartridge are NOT listed.
The scoop is nothing more than a volumetric measure, not unlike every mechanical powder thrower now on the market.
As with any other means of measuring powder, a scale should always be used to verify that what you're expecting is actually what you're getting.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.