Q: Why does the kit include billet rear camber arms?

Because lowering a Model 3 adds negative camber at the rear, and uncorrected that wears the inside edge of your tires. The CNC 6061-T6 billet camber arms, running sealed OEM-grade bearings, let you bring rear camber back toward factory to protect tire life, or add negative camber if you want more grip for track days. They ship preset to factory length as a baseline, so your shop has an easy starting point at alignment.