Plastic bottles have bumps on the bottom for structural integrity and to help add support to the bottle because of the carbonation.
Soda bottles have bumps on the bottom for structural integrity as well as to keep them from being crushed when other bottles are stacked onto them.
It helps to spread out the weight of the bottle.
The “bumps” on the soda bottles and other bottles add extra strength to the bottom of the bottle to withstand the pressure of the carbonation.
Other shapes of rounded bottoms could work, but these keep the ability to stand upright.
Water bottles have small indents to improve the stability of the bottle.
The bumps or the corrugations in the foot of the bottle increases the strength of the bottle.
Take a sheet of paper and fold it. You can easily fold the paper.
Now bend any one side of the sheet and try to fold it.
You will now feel some resistance. Now, roll the paper into a pipe and try folding it. It will be little hard to fold the paper pipe.
Bending any material increases its strength and rigidity.
This is because; rolling the paper increases its moment of inertia around the bending axis.
Soft drinks are served chilled. When a liquid is cooled, its volume changes and the bottle must not buckle because of this change in volume.
The corner or the tip of the bump can slightly expand/contract without breaking which takes care of the change in volume of the liquid.
One more reason is that most of the soft drinks are pressurized by filling gas; the plastic bottle must be able to withstand the pressure.
It should also absorb the shocks when you accidentally drop down the bottle.
Next time you buy a soft drink, try crushing the bottle.
You can easily crush the top part but you will find it hard to crush the foot of the bottle.
This foot also improves the stability of the bottle.