Adult birds do not need to have food available all day. Some birds will overeat (dark cornish) and constant food invites mice, rats, and small birds to come and dine (assuming they can get in).
For faster growth of chicks it is better to have food available all day. Digesting food also creates heat keeping them warm.
I would feed adults in afternoon so that they would graze more and eat table scraps in the morning. They also spoil less food when hungry. It also makes them easy to train if you call them when feeding (if they are free ranging). Our call was here cut cut cut cut. The flock would come running to us when called.
The crop is a safety strategy for gallinaceous birds. This includes pheasants, partridges, grouse, quail, etc.
Food often grows in open areas (corn fields) where predators can swoop in. This way birds can run out and fill the crop in 5 mins and run back to cover.
It also lets them fill up so they can digest the food (creating heat to keep them warm) during the night.
I know in grouse hunting (ruffed grouse commonly called partridge) the birds will poke around carefully all day making them hard to hunt. At around 4pm they will be found in the food areas filling up before roosting before sunset (5-6:30 pm in the Canadian autumn). The best strategy when hunting is to open the crop of the first bird found for clues to what they are eating so you can hunt where you find these foods.
Thus the purpose of the crop.