Seasoned pool operators know the triad of great pool water: filtration, circulation and chemistry. Without effective filtration, there will be problems... and filter channeling undermines filtration.
When sand filters become overloaded or otherwise compromised, water must still find its way through the filter. The water pressure can eventually force its way around the sand (or a channel through it), creating a new path of least resistance for water to travel. Filter channeling is a problem that almost guarantees cloudy pool water, as the channel itself is like a highway for larger particles to travel and bypass the filter media.