A sleeveless T-shirt is great for a warmer day or when working on a messy project, like changing your oil. Whether you wear it on its own or under a dressier shirt, having a few around provides you with great layering options.
Perhaps you like to show off your arms and avoid tan lines. Grab a sleeveless T-shirt for the gym or select one with your favourite cartoon. Your arms do not want to be encumbered.