A post-meal workout helps the body pull in carbohydrates (i.e. glycogen) from the bloodstream and directly store them into muscle. This helps you develop greater endurance (as muscles have more stored glycogen that helps them work harder, longer). Carbohydrates (i.e. sugars) that are pulled from the bloodstream and stored in muscle after a meal do not end up being stored as fat in the body's fat cells and therefore help towards a body that is leaner. The greater benefit by far however is that post-meal activity helps the body's response to insulin which means you can more easily access stored energy in the body and are less likely to develop type-2 diabetes.