Lot of it dates back to the days of loose hay (not loafs), you had no choice but to have your hay corrals in the field because you had to more or less drag and push it all into that spot.  We are slowly moving all ours out of the fields, more because they get irrigated and ruin even more hay that sits for over a year.  If you want to irrigate a high spot in a field build a hay corral on it and it will be the wettest part of the field in no time.

Now if you are talking about having hay in a field other than all around a house, before we all got bale beds you didn't have an easy way to haul that hay, in some cases many miles, back to the same place it was put up to feed the cows. 

If you want to know how much hay someone puts up in a field look at the number of hay corrals and placement, it is actually quite interesting once you get the hang of it.  Just moving from loose to baled hay we have easily cut our hay corral numbers in half, still know a few guys that put up loose hay and you'd be amazed how many yards there are in a field if you aren't used to it.