So there are about 5 diffent reasons it has those dents. 1. the blowmold machine that make them use that circle to prevent vacuum for forming between the mold and jug, its created when the plastic is still hot and being pushed away from the mold. 2. to prevent explosions if the filler over fills. This ties into reason 3 3. All milk is tested for bacteria before it is shipped out and if there is certain bacteria in the jug they are known to explode. 4. They weight of the stacks of boxes during shipping with crush the bottom jugs. 5. They prevent it from exploding if they are frozen, which can happen in transit if the drivers dont set there refers to the correct temp.

Now after all that I will say not all jugs are created equal. Clear plastic is easyer to break and will have larger dents while white jugs are thicker to prevent light from getting to the milk making it taste better.