One advantage I've found is that it can help you widen your power band. On downhills or with a tailwind, your gear will be too low and you'll be forced to spin a higher cadence than you're used to, thus building supplesse. On uphills or with a headwind, your gear will be too high and you'll be forced to muscle up, thus building strength. Translating to a geared bike: when the group surges, you'll be able to spin up to match the surge and then drop a cog. It's much smoother. Or digging hard out of a corner or powering up a short steep hill without needing to drop onto the small ring.