The medical definition of hernia is "protrusion through an area of weakness".  They usually involve a piece of intestine and sometimes this can become "strangulated" which is very life threatening.  You don't want to leave it until it becomes an emergency and he's got colic and could then get septicaemia from it too.  If done in a non emergency situation it is a much simpler op and the recovery is much more straight forward.  Ripleyuk is correct, the muscle does need to be sutured to prevent the hernia recurring.  Check with your vet re the cost as in our area it's nowhere near £600 for gelding.

I have never heard of hernia's just going, Our foal had one as you have described, we had his operated on at the same time he was gelded, the muscle needs to be closed over the hole, we had no problems after his op and you would never know he had a hernia.