With sous vide, precise temperature control in essential because of the low temperatures - and the fact you're trying to get the meat to a specific temperature. A steak done to 135 degrees will have a very different character than one done to 140 degrees. The beauty of sous vide is that no matter the size of the steak, you just need to leave it in a bath of the proper temperature long enough. How long is that? Well, if you can come up with a reasonable estimate, just leave it in there for that long, plus 1-2 hours just to be sure. That's not practical advise, but just something that sous vide allows you to do. As soon as you get above those temperatures - into frying temps, the prospects of overdoneness loom ever nearer. Now it becomes less about the specific temperature and more about understanding the characteristics of what you're cooking. Different cuts/shapes/thicknesses will take different amounts of time. You have to be be the arbiter by judging how the meat is reacting. The tools that will help you best with this are an independent oven thermometer, an IR thermometer, a Thermapen, and a practiced finger. If you lack any of these, put off any other further purchases until you have collected the set. For sauces, fine temperature control will be of no use whatsoever unless you're doing some sort of exotic molecular gastronomy sort of thing.