Bottom drains are a must for a successful Koi pond. To maintain proper water quality you need to get the fish waste into the filter so it can be removed.
The vast majority of solids will sink out to the bottom of the pond in time, where ideally, they will be sucked into the bottom drain and fed to the filter systems where they will be extracted from the Koi pond system.
As a general rule, one bottom drain for the first 5,000 gallons and then one drain for every 3,000 to 5,000 gallons thereafter. Of course, the size of your pond will have a lot to do with this - as will the flow rate you want through your filter systems.
Bottom drains or the pipe work from the drain to the settlement chamber should not be too long. This is because there is water flow through the pipe work, the heavier solids tend to settle out and can build up in the pipe work itself.
Many pond builders have found that adding a bottom drain helps keep a pond cleaner. Fish waste and other much that accumulates in a pond can be easily removed by a bottom drain.
Ponds that are over 2' deep can become stagnant without a bottom drain.