Using Dy'on blinkers helped me a lot with my 7 years old who used to be very distracted by the slightest thing at home and at the show, it keeps him more canalized and focused on work. The good point about the Dy'on design is that it is approved by every national federation and the FEI, you can put them at every show. :)
There are two reasons. Horses, when they are harassed by the flies that land around their eyes, naturally shake their heads to get rid of them. These head movements are manageable for a rider and for horses. Blinkers help reduce this problem.
You should know that horses have a vision system for herbivores hunted by carnivores. The eyes are placed on the side, not in front; They have a visual field of 340 to spot the danger.
So, on a race field, in competition, or in the traffic of a city, the opportunities to take fear are countless for the horses. By putting blinders on them, one limits their visual field to what is necessary for their forward progression, and one avoids that they take fear of everything that happens to their side or towards the rear.
I hope this helps,