In the computer age, people have become extra-busy, and sleeping less. So to keep sleepiness at bay, caffeine and energy drinks have become a must for people in real life. Reflected in Second Life, the machines are mainly for decoration, the cans they dispense for show.
But just suppose someone in the Grid came up with a vending machine that dispensed cans of which holding one cut down on one's lag-time. He'd make a killing.