Why are the bottoms of plastic soda bottles shaped that way?

They’re… um… crenellated? Here’s a picture of one from below. (nevermind they made a wind chime out of my example).

Is there a particular purpose for that shape (other than making your drink precariously perched and subject to tipping at any moment)? Is there a name for that shape?

And size of the bottle seems to matter not a whit. Small, medium, large, different brands even. If the bottle is plastic and contains soda, the bottom is shaped like that.

Plastic bottles for water, fruit juice, tea, sports drinks, etc. are not crenellated.