There is certainly a few folks that would have you believe that DI will only work if you're moving, or is useless unless you are moving. The image below, is from a short trip to a local lake today. My speed is 0.0mph, and as long as I see the horizontal lines on my screen (crappie), I know I'm in the right place. Dropped a rig down to the bottom, up a couple cranks and wait. They were slamming it. The fish were suspended from the bottom up 5ft, and there was about a 10ft wide patch of them. Once you drifted outside the patch, the screen was blank.

2d sonar, would not have shown anything close to this.