That small circular dent on the side of a plastic milk jug is a deliberate engineering choice, not a cosmetic quirk. As the jug is filled, the liquid inside creates internal pressure; the dent acts as a flexible zone that can bulge outward or pull inward as temperatures change or the jug is stacked, helping prevent cracks, leaks, or warping. If the jug is dropped, that recessed area can deform slightly to absorb some of the impact, making a messy spill less likely.
Manufacturers also use the dent to strengthen the container without adding more plastic, reinforcing the structure while keeping the walls relatively thin. This reduces material use, lowers production costs, and cuts plastic waste. The design even aids recycling by using less resin and creating a more crushable, compact shape. That quiet little circle is doing far more work than most shoppers ever realize.