Well if its there, mine will guzzle it; so as he's in at night (sweet itch) and/or during the day when its hot/flies, I tend to give him lots of smaller haynets if he's in, so he can't just settle in and guzzle it all down fast.
I also tend to (or try to!) fill him up with Dengie Hi Fi Lite & Fast Fibre; so he's hopefully not feeling so hungry before he has the haynet.
The benefits really depend on the horse but:
1) keeps the gut moving
2) keeps the horse warm in cold weather
3) helps prevent boredom
4) helps prevent / minimise hunger / boredom / stress related vices.
5) all the above mean that the horse maintains / gains weight easier.
Horses will eat as much as they can eat but you can control that amount by feeding it loose of the floor / netting it accordingly.
I did an experiment once with some horses that were on restricted hay. I gave them half a bale, on the floor, the first night and they ate it all so the next night i gave them more, again it was all gone in the morning. The third night i gave them a whole bale and there was about half of it left in the morning. If horses are restricted they will eat until it is all gone but if it is adlib they adopt a more natural eating pattern of eating for 16 -18 hours per day and don't feel the need to gorge. Interesting i thought.