Headshaking, pulling when on hacks etc and allergies ( dust, pollen etc). You can also get a net muzzle can be used for biters or a sun visor which attaches to the noseband on bridle or headcollar.
A friend of mine in racing once used a nose net( sometimes called a net muzzle but be careful as two types exist, the one's that look like a bag and those which just fit over the top part of the nose, usually a nose net but can be called muzzle by some) it was used on a horse that used to headshake and was a strong puller ( it was a front runner) the net helped him to settle and even though he set the pace in his races the application of one helped him to win. Run for Dante was its name.
Hope that helps
Cazx
I use one of Murphy in the summer - he reacts badly to the midges and snatches his head and snorts, not majorly but doesn't do it all with nose net on.