It’s pretty ridiculous, isn’t it. I can see why teams need a decent reserve driver- to test in the simulator, drive in free practice, keep the regular drivers on their toes, become aquainted with a team they are likely to race for in the future. Nico Hulkenberg and Daniel Riciardo are two who meet those criteria.
But, as Klon suggests, I think money often plays a large part too. For a team like Sauber, with two inexperienced race drivers, to choose a rookie as a third driver is quite inexplicable. Except, of course, he’s Mexican, and it doesn’t take a genius to work out that Sauber’s Mexican backing might be conditional on them running two Mexican drivers