I know many artists don't do this but when it comes to commissions it is something I always like to do. A test model is simply a model that you paint alongside the actual commission which allows you to test paint formulas and techniques as you go so that you can make sure that the commission piece is as perfect as possible. It is something I have always done but only today did I find out how much having one of these models is so useful!
I have been working on a commission piece for someone who wanted a portrait of a horse they have recently had to see go. It is a nasty difficult colour so having a test piece was rather useful. But it proved even more useful when it turned out that my second mix of paint for the second coat was a little bit too umm...'orange'. The poor custom in question looks like he's been tangoed! Luckily I was able to modify the mix before putting it onto the commission piece and hence ensuring a nicer finish.
For all of you who don't currently use them I highly recommend it! It helps you get precision with your colours and give your clients the best possible workmanship.