Let's say you are painting without first making a few thumbnails (to plan your composition) and without making a preliminary drawing in black and white (to understand the values). Let's say you gave nominal thought to your palette. You probably grabbed the colors you always use. To complicate matters, your time is very limited, about as much time as a professional model can hold the pose (20 minutes) without needing a break. If you put all of these conditions together, you've got yourself a study.
Artists engage in this practice for various reasons. Since materials have always been expensive, the most commom use has been to "try out" how various combinations of the above elements will work out together in a larger painting. A study can be used because its faster, looser style can capture the subject's character or the atmosphere of a place. You can use it to train yourself to catch the essential elements of what you're painting during the first few minutes.
This last purpose is why I put myself through the pain of sketching with oil in this brief period. It can be frustrating, because the medium was not created for this purpose. Try layering oil paint that has not dried. Try wiping off mistakes you've made in a strong color. Try finishing in 20 min. using a format larger than 11 x 14. Impossible! But so many decisions are made during the first 20 minutes of a painting, that it is worthwhile to learn to make them efficiently and successfully.