Harness horses often have blinkers or blinders - leather flaps positioned outside the eyes. Why is this?
The traditional reason for blinkers is to prevent a horse from spooking at the vehicle it is pulling. Some people do not believe they are necessary, and you might occasionally see a horse pulling without them - often an experienced, finished horse. Blinkers also discourage a horse from running backwards, which can cause it to hit the carriage and cause a wreck.
Blinkers are also sometimes seen on racehorses - this prevents the horse from being distracted by the other horses in the race, which can make some horses slow down. In some cases, blinkers can also fool a horse into thinking he's at the back of the pack when he's at the front - some racehorses slow down when they hit the front, either because they don't want to go first (a common aspect of horse psychology) or because they think they've already won and don't need to put in more of an effort. Racing blinkers are attacked to a hood rather than to the bridle and are generally decorated in the owners' colors.
Blinkers are seldom seen on normal riding horses and are illegal in most saddle competition. You do occasionally see them on a particularly spooky horse on the trail.
Harness horse with blinkers. Note also the brass "fittings" on the bridle. Image source: Alex Proimos via Wikimedia Commons.
Racehorse with American style racing blinkers. Note that these restrict vision much less than the harness blinkers above. Image source: Maryland GovPics via Wikimedia Commons.