The temperature is dipping, and you're wondering if you should put a warm winter blanket or 'turn-out rug' on your horse. Chances are if the weather is 'wintery' but not windy or wet, your horse probably doesn't need a blanket. As long as your horse has access to good quality hay and fresh water the heat generated by its digestive system, and the natural protection of a thick winter hair coat will probably keep it comfortable in weather that would send you running for a warm jacket.
Although some maintain that you should never blanket a horse, there are some situations where it may be a good idea. Older horses or horses that may have trouble keeping weight on in good weather will burn a lot of calories to keep warm. These horses will benefit from extra feed and the extra protection of a wind and waterproof horse blanket. All horses will benefit if there is some sort of windbreak or run-in shelter available to escape the direct brunt of the wind, rain or snow.
If the weather is very wet, the natural loft of your horse's hair coat will be lost, and just like a wet down jacket, will not keep cold out. If it is very windy, body heat is blown away. Horses can be left un-blanketed in very still, cold weather -4F (-20C), and they may be fine. However, add wind-chill or rain into the mix, and you can quickly have a shivering horse.
There are hazards to blanketing. Within a group of horses, one often makes a game of 'destroy the blanket'. That's frustrating for you, especially if you've just saved up to buy that extra-special blanket for your horse and after the first day, it has big teeth marks in it. It's also a hazard, because a ripped blanket is easier for a horse to be tangled in. Horses can be caught in loose belly or leg straps too. Coolers, sheets and stable blankets aren't suitable for outdoor turn-out. They often lack the straps and fasteners that prevent outdoor blankets from shifting.
Poorly fitting winter blankets can severely chafe or cut a horse's skin. If winter blankets aren't made of breathable fabrics, the horse can sweat underneath and become uncomfortably wet. Likewise, horses left blanketed when the weather turns mild will be uncomfortable. Some blankets are made with layers that can be used separately. These are convenient, but could be a problem if the layers shift.
If you often deal with wet weather, it might be handy to have two blankets. If one blanket becomes saturated, you'll have an extra for your horse while the other dries out. Wearing a wet blanket is as bad as wearing no blanket.
Whenever I approached my horses carrying their blankets, they'd gallop to the furthest corners of the paddocks and be difficult to catch. One night, a sudden storm blew up with driving sleet and wind. To my amazement, the horses saw me coming and quietly waited as I put each blanket on. Clearly, they saw the blankets as relief from the bad weather. I learned you have to use your judgment as to whether or not you feel comfortable with your horse wearing a blanket. Let your horse's behavior and physical condition be your guide.