If you have a theory about such things, yours can be compared to a substantial list of possible explanations – bearing in mind that experts in the field are as yet undecided as to which one(s) might be correct. They include (but are not limited to) :
● Having the function of making the bottle less likely to topple over.
● Allowing bottles to be more easily stacked end to end.
● Taking up some volume of the bottle, causing it to appear larger for the same amount of wine.
Photo Credit: Aurélien Mole @ Wikipedia