Anytime your sleeves might get in the way or dirty or caught in a moving part — roll ‘em up. Thus it is the universal “men at work” style. Examples of this include unclogging a toilet, or perhaps rescuing a cat from a tree (though in the latter case you might actually leave the sleeves up to avoid scratches on your skin).
When it’s very hot and stagnant, rolling up the sleeves lets air flow directly over more of your skin. Every little bit helps. While the appropriateness of rolled sleeves varies by workplace (see below), anytime you’ve got direct sunlight on your skin, rolled sleeves are acceptable.
They’re also nice when you work or live inside a place where the temperature fluctuates a great deal, as it’s nice to be able to roll them down when the A/C’s blowing on you, and roll them up when the room gets stuffy.