It's main purpose is from the level 68 trait. If you still have 3 AF stacks and are approaching 30s left on your AF CD, you want to get rid of them to make use of the 20% proc. Assuming there's very light damage going out you can just ED 3 times, but if you know there will be damage coming soon you might as well Excog and "stock" that healing.

Playing around a 20% proc chance might seem silly, but it's all about efficiency, getting a quickened aetherflow proc at 3 sec left on your AF CD is just a big waste.