It's basically an event that doubles your credits whenever you happen you increase your refines.
the more you increase, the more credit you get. It's basically the fastest and easiest way to ugrade your refines. It's why people save up until the event spawns.
Also, you're best bet is to buy med refines from the group shop and other events like monthly and such.

It's best to work on one at a time. The more you spend on one, the better chance you have it of upgrading faster, then you can go to the next. It's best to stop around 7, because beyond that you'll need advanced refines.