Moisture
A ripe cherry is roughly half water when it is picked. Drying at origin brings that seed down to somewhere around 10 to 12 percent, and moisture is one of the properties the specialty grading standards actually specify and test for1.
Below about 10 percent and the coffee has been dried too far. It goes flat and papery and you cannot get it back. Above 12 percent and the lot starts carrying a mould risk in storage, which is a risk to the person drinking it, not just to the flavor.
But the reason we measure it is not compliance. Water carries heat. So the moisture reading is the first thing that tells us how this particular coffee is going to behave once heat starts going into it.
Density
Density is mass relative to volume, and it is mostly the fingerprint that elevation left behind.
A dense bean needs more energy to get heat all the way through its structure. A softer, lower density bean is far more sensitive, and too much heat or airflow will scorch the outside or bake the inside before the middle is ready. Kenyans and Ethiopians sit at the hard end of that scale. A soft Brazilian sits at the other.
Those two beans cannot go on the same profile, and density is how we know that before we waste a batch finding out.
Why we do this on every lot
Because none of it is constant. The same farm, the same variety and the same process will still give you a slightly different coffee from one harvest to the next, and a lot that has been sitting in storage for six months is not the lot that arrived. Measuring is just refusing to assume.
Try this
You cannot measure density at home without a kit, but you can see it. Put a few beans from a high grown washed coffee next to a few from a low grown natural. The high grown ones are usually smaller for their weight, harder, and the centre crease is tighter.