Because balance druid damage spikes up when they enter eclipse, and being able to control exactly when you go into eclipse is super helpful for applying pressure in a pvp setting. Imagine you're in a 3v3 team with another dps and a healer, as it currently stands you basically have to wait for the druid to start an eclipse in order to even begin considering making a target call and even then, you have a very short window to make a target call. Now you can plan a target call, the druid can be ready for it in 4 seconds max, and get into position while getting prep'ed. If the druid needs to try to keep pressure up a bit longer than the length of one eclipse, they can do a four second rests between and get back into eclipse, such as starting in solar, then when solar ends AC to lunar and pop starfall. Maybe toss in a cyclone or solar beam somewhere in there to slow heals down.
Granted AC on the move isn't a strong button, but it's something that is needed kinda button. A button that many raiding moonkin would actually love to have, so we don't lose eclipsed dots in order to use our "movement dps mechanic".