Why drinks cans narrow at the top and bottom

Why are beer cans tapered at each end rather than being straight cylinders?

To save metal and strengthen the can. The lids and bases must be much thicker than the walls to withstand internal pressure, so making them smaller in diameter saves aluminium and cost; the domed, tapered base also resists pressure better. The taper also lets cans stack securely on top of each other, and the narrower lid makes the pull tab easier to place.