All ponds, at some time or another, need a good clean. Decomposing debris lies on the bottom of the pond. This releases ammonia, nitrite and nitrates into the water. When exposed in high levels, this can be harmful to your fish, and in some cases can prove fatal.
Dirty water spoils the look of the pond - your water should be clear so that you can see right to the bottom. Plants need to be kept in check so that they don`t become too overgrown.