Goods production buildings can be upgraded to produce more of a given recipe at once. For instance, if you upgrade the Windmill, you can produce a double batch of Flour at once. Do note that it still costs the same number of input goods – you’ll need 4 Wheat to produce 2 Flour.
In addition to producing more with a single action and keeping your buildings producing more while you’re away, upgrading buildings has another benefit. The increased production actually completes faster per unit! For instance, producing one unit of Flour takes 20 seconds, but producing 2 takes 35, and 3 takes 50. It doesn’t make that much difference for something like Flour, but those time discounts add up for higher level goods.
The only downside? Upgrading buildings takes Keys which are rare or cost you actual, real world money.
Check around the farm for Hope Chests, and tap them to get more keys. Sometimes you’ll get them for completing quests in the Farmer’s Almanac. Whatever you do to get keys, though, I highly suggest you save them for upgrading your buildings!