I look at creating a study before a major painting purely as practice and a way of working out colours, values, composition, and background. Its no different than a pianist practicing a piece before a recital or knitting a swatch before starting a full sweater.
I like working out my palette with a study before I tackle a full piece. It takes a lot of the guesswork out of things if I can work out my mistakes on a small scale before committing them on a large scale. Its cheaper also, as I'm not wasting materials.
My studies generally don't take more than an hour or two to complete. They're not finished pieces and never will be, but they provide the template for the painting that will take place next. The study acts as a guide when I do my full piece in terms of colour placement, values, etc.
I don't do a study for every painting that I create, as I don't want to lose the spontanaiety that comes with starting fresh. I do use studies when I have an important painting to create or a commissioned portrait that must be correct.
Often beginning artists want to achieve results in the least amount of time so completing a study doesn't appeal to them. It is a time saving exercise in the long run. It also relaxes you by letting you work out potential problem areas ahead of time in prepartion for the painting you'll do. And once, your painting has been sold or delivered, you still have a study for your files.
I'm taking part in a portrait swap on WetCanvas, which has become a bit of a ritual for this time of year. You add your name and are paired up with another person whose portrait you create and they in turn create one of you. They can be anywhere in the world which adds to the fun of it.
This year, I was paired with someone in Newfoundland, the same town more or less. So we decided that we'd take the opportunity to do some drawing from life and take some photos then work from there. This morning I sat for him and tomorrow he will come to the farm to sit for me.
We had exchanged photos previously and I did a warm up sketch which then turned into a study in skin tones as I most likely will do a watercolour portrait. Of course, I didn't use watercolour paper for the intial drawing so the water buckled the paper. That's ok as its only a study.