I ask about the sap brix because (and this goes for almost all plants) if the plant sap brix is very high, insects don't want to munch on the leaves as it's indigestible to them.

A sap analysis will tell you what nutrients (macro and micro) the plant is actually uptaking.

Lets say a soil test says you have plenty of phosphorus. The sap test on that plant doesn't show enough phosphorus. That means the phosphorus in your soil is mostly unavailable for plant uptake.