Why do riders wear hairnets?

If you look at videos or pictures of an English show, you'll see a lot of the lady riders wear hairnets.

Rider's hairnets are generally a similar color to your hair and they're designed to keep long hair neatly out of the way without using a pony tail holder, which can be annoying with a helmet (I personally just use the holder, but I've never had that issue).

It's also, well, tradition.