Beverage cans (usually made to contain carbonated drinks) are designed to hold their shape while pressurised, in fact non-carbonated drinks are pressurised with nitrogen to give their cans strength while being shipped and stored.

Cans must be stackable; that is it must be possible to place one on top of the other in a stable manner. If the bottom of aluminium cans were not made in this concave shape, and were made flat instead, they would tend to bulge outward slightly from the pressure, thus making stacking of the cans impossible. Such methods as adding plastic sleeves to the bottoms of cans is not really feasible, as the cost per can must be kept down to an absolute minimum when such a large volume of product is produced. (This is somewhere round about the trillion can per year mark).

Here in England, and indeed all the other European countries I have visited, our plastic beverage bottles are not designed with a concave bottom and a false base, but are moulded into a pentagonal curvy shape which gives it both strength under pressure, and the ability to stand up on a flat surface. (I can't really describe the shape, but I'm sure there are pictures of it somewhere on the www.)