We throw flat ground to work on mechanics, conditioning, and spots. Our younger kids will throw a 3-1-3 (3 fastball spots in a row, 1 change up spot, and 3 knuckle palm spots in a row) our older boys throw a 5-3-5 usually. We do gun guys off flat ground early on just to see where they are at, but we've never done it from flat ground and the mound in the same day. Typically our flat ground work is 3-4 mph slower than the mound.
I will say I believe throwing spots from flat ground is great for pitchers. We do it a lot since it is less stressful on the elbow. When you have 4-5 guys on a middle school team that can throw an 80mph inside fastball spot and can locate a knuckle palm away it makes it a lot easier to call pitches!