Practical Yet Functional
I watched a documentation about modern bottling and packing practices and they had an interesting segment on how bottles are actually designed.
An arch is a strong structure. Imagine taking an arch and spinning it around, you get a dome which is also a very strong structure. It's good at resisting forces from inside too.
A bottle with a dome can better resist pressure. Soda has pressure because the bubbles are CO2 gas which can dissolve in water, but getting enough into the water to make it fizzy takes cold and pressure. When you do get enough in there, it doesn't like to stay in the water. Keeping the water under a bit of pressure can help though. We have glass soda bottles without curved bottoms, so what's different? A curved bottom means the bottle can be thin, and it's easy to make.
Old plastic soda bottles had a plastic cap glued onto them so they could stand up with the curved bottom. But this is extra material and extra steps. I would also bet the old bottles were harder to recycle but they were phased out before I can recall recycling becoming the norm. I believe someone figure out the 5 pointed bottom would work well enough because we are not talking about crazy high pressures. I believe the bottom of these bottles are also a little thicker than the old ones. But the saving in materials, fewer manufacturing steps, and making them easier to recycle beat out the increases in price for the new process.
Interestingly enough, many of the "aesthetic" features on bottles are for functional purposes rather than looks. Carbonated soft drink (CSD) or Pop bottle bases (petaloid) are shaped this way because of the pressure in the bottle. The ideal base for a pressurized bottle would be a sphere, however this is not practical for keeping your pop upright, but if you ever look at packages such as a gel shaving cream dispenser you will notice that there is a cup fit onto the bottom of the bottle. This is because shaving cream requires even higher pressures and so Actually does have a spherical bottom and has a base insert that keeps it upright.
Another example of function rather than form in plastic packaging is sports drinks (anything ending in an "ade").these bottles are called hot fill meaning that they are.... ta da. Filled hot, so the inset panels that you see on these bottles are to accommodate the loss of pressure once the liquid cools down. These bottles start with the panels popped out and during filling they suck in.
There is a lot of interesting things about the plastic packaging world that many of us take for granted until we understand all that goes into it.
If you want to see the ability of a soda bottle to hold pressure in action, google "soda bottle water rocket" (my daughter loved watching the cute one with the bubbles added to the bottle contents). If your launchpad can fit the mouth of a water bottle, you can see how it behaves under pressure in contrast.