A post workout protein, although something like a Whey Protein Isolate or Whey Protein Blend can be used post workout, this category is catered towards a blend of protein and carbohydrates.
A post workout can be used in any phase of diet, provided you are monitoring food intakes. As there is generally a 50/50 blend of protein to carbs, you are able to maintain muscle fullness in a cutting phase, as you replenish glycogen stores when the body is craving it. This happens as the body "opens a gateway" as such to the muscle, as the pancreas releases insulin to enable to storage of nutrients such as protein, carbohydrate, amino acids and creatines.
This is extremely beneficial when trying to maintain as much size in a cutting phase of dieting, or especially when looking to grow as much as possible in a growth phase. The main benefit being provided you monitor overall food intake, you won't gain massive amounts of fat.