>>11986841: Mid-pack, may need to shift to keep up with a surge, may need to feather the brakes
2: Working harder to keep up with a surge or ascending a short climb
3: Chilling mid-pack, sitting up to take a break and a look around and pull from your bottle
4: Your category has been temporarily neutralized by the referee; break time!
5: Proper hand placement for a sprint, fast technical descent, or otherwise working at maximum
..among other reasons.
They're all valid, they all have their uses, and if you're not using all of them at one point or another, then I wonder how far or long you ever ride, racing or not.