The point behind the rounded bottom was to ensure that the bottle was left on its side so that the wired down cork would not dry out and shrink allowing the contents to loose carbonation and/or evaporate. The typical rounded base bottle was made of thick heavy glass and used for carbonated soda, mineral water, and in particular, ginger ale. They were mold blown with the lip being finished by hand.
These type bottles are commonly referred to as "round bottom sodas" or "ballast bottles" since it is believed that many of them were imported from England as "ballast" (weight) in ships returning to the United States.
http://www.bottlebooks.com/questions/common/round_bottom_sodas.htm