It does exactly what it's name implies, it reflects the light / heat more upward. I usually take them off my centurys just because you really don't need it if the globe flows fine without one and it also will brighten up your pinhole light too.. But leaving it in will also help lazy globes flow a little bit better so it's a matter of personal preference. It can also be a pain in the ass to remove stubborn bulbs with the ring on so that's another reason why I don't keep them in.