You know, the two circular indents on the sides opposite the handle, what the heck are those things? The only thing I can think of is they have some structural purpose, like the circular windows on an airplane.
Just a WAG but it may help keep the jug from bulging outward. When things made of thin, semi-rigid materials like sheet metal or plastic are made with large flat areas they tend to bulge and buckle easily. You can demonstrate this on a car with a large flat roof. Easy to press down and buckle the metal slightly but very hard to do it on a compound curve like the corner of a fender.
FWIW Airplane windows are round to avoid cracks in the metal structure. Corners can form a stress riser and making a round window more evenly distributes the stress so there is no one place a crack is more likely to start.
Previous thread: Milk Jug Indentations
my bad, I think my searching skills need refinement
Dah!
About two weeks ago, I wondered what might be printed under the label of the bottle of Gatorade. Hadn’t noticed till then, the bottle has a series of large figure 8 indentations.
Each time I have been in a grocery or C-store, my eyes have gone to plastic bottles, water, juice etc. Every one has some series of grooves, circles or indentations.
Well, I know why they’re on apple cider. I left a half-jug of it in my fridge and left town for a few weeks. The bottle bulged all the way out. If those indents weren’t there, I may had to have moved.
And they help when you drop them too, of course.
dtilque, its okay if you didn’t search, how many times would anyone expect to see a topic on milk jug indents? 
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I couldn’t reach them but seems to me they would know. They have a web site doitwithdairy.com