Though many battery-powered devices these days have a rechargeable lithium-ion power source built in, portable electronics originally used disposable lead-acid batteries. The most common type in the late 20th century as personal computers became popular was the AA battery, a cylindrical cell a bit smaller than your last finger. These and other batteries have a positive and negative terminal, and had to be orientated correctly to release power into a circuit. To make this easier to do, the positive terminal had a small bump to differentiate it.
Today’s laptop, phone, and other device batteries are generally just rectangular or conform to the device’s form factor, and as such no particular shape says “battery.” As a result, we have retained the old shape of a cross section of a AA-type battery, with a positive terminal at one end. Coloration indicates the battery level of the internal battery, though there was no such indicator on AAs.