The trenches create drop off. Those areas become hot spots for our predator fish to hide. It's where they will find the most feeder fish. The pond has to have place for the feeder fish to hide and escape the predators. This is what log are for too. The edge of those ditches is where the best fishing is going to be. Same as the edges of the log piles.
Since I have channel cats in my pond, along with large mouth bass and copper nose blue gill, I dug one pit in my pond that is about 20x60 and 12 feet deeper then the rest of the pond, which is around 12 feet deep in that area. That hole is a hot spot for catfish, and the edges around it is one of my favorite spots for bass.
When you dig the ditch, pile up the dirt into a big pile. This will give you two areas of habitat. Both work the same way, it provides a place for feeder fish to hide, and predators to hunt.