Keeping tabs on the beasts innards.
Let me tell you, I love gauges. I love knowing what is going on with every part of my truck. One of the best ways to know what is going on inside your differential is with a diff temperature gauge. Since part of this build is longevity a gauge is a small price (literaly at only $45 dollars) for an early warning if anything is going south.
Why is the sender so high you ask? It is placed at the top so that the sensor takes its reading from the hot oil that is slung off of the ring gear. This will give a more acurate reading of how hot the oil is actually getting inside the diff, not just the average temp of the 'pool' of oil.