No delay means that Stalkers can’t blink away. No overkill means that all of your Roaches won’t waste extra shots when you focus fire, say, an Immortal. Additionally, because it’s melee, it isn’t affected by Guardian Shield or Point Defense Drone and is packaged with a different animation. It does, however, still count range upgrades.
How did I not know about this? And I thought that the Roach surround was just to stop kiting. I now have a good reason to micro my Roach-Hydralisk balls even more than just getting them all in range.