It wont necessarily dry out but it will become stagnate and trap the stuff in the filter only ..It also insures that the fresh media is in your system to clean and polish your water , Fresh carbon will help trap any solids that were not picked up due to the filter down time as well as the fact that it really is just sitting and rotting with out the water flow ... Water changes also dilute your ammonia and fish poo that are in the tank breaking down . When you have a system breech a few of the most important steps are :
Keep the fish from stressing
Keep the water polished and pristine this helps prevent them from breaking out in ich or other stress related diseases which are many in reefing .
Changing the media insures that all the heavy pollution is going through fresh media , and polishing off your water .
The water change also dilutes the polutiants in your system
10 gallon ~6.5 gallon custom