Nosebands are sometimes used for decoration only, which is fine, they are not adjusted inhumanly, meaning they are a loose-fit.
Historically speaking, why did they come about? With western bridles we have the throatlatch to help hold the bridle on, if you ride through trees or other obstacles that could rip the bridle off (I have had this happen! And my throatlatch has been looked at as a more important feature ever since!), but a noseband is just there, most are not attached to the bridle at all... and most often they are used brutally, showing extreme ignorance on the user's part.
Interesting subject...

Ours are trained with the nosebands. The idea is, if they are just in their headpeices then they must respond to being led. If we are stood then they are allowed to eat (as they might be allowed too if they were travelling long distances). Once their nosebands are no, they do not eat. Even if they are stood for a few hours on grass, with their nosebands on they will not eat.
It's just our way of differentiating for them when they are going to be worked and when they are allowed to eat and wind down.