Some horses are very focused as they run, but others are not. They are distracted by challengers, or even by random events or features of the scenery around the track. They run past a bush and a bird rustles the leaves. Instead of perceiving a bird, they think there is a predator lurking inside and they bolt to the side.
Horses that are easily distracted are anxious and performance suffers as a result. That is why blinkers were invented.