A Windmill is used to make Barley Flour and Wheat Flour. Those items are used to make various things using the profession Cooking. They are also used for various Part-time jobs. I have seen Caitin of the Grocery Store in Tir Chonaill asking for those items mostly. Endlyon at the Church in Tir Chonaill also asks for barley/wheat stalks.
Using the WindmillEdit
To operate the Windmill, talk to the NPC standing near it. There will be two options, one minute and five minutes. For the making of multiple sacks of flour, I advise five minutes. For the making of one sack, one minute is fine. After paying for and activating the Windmill, walk up to it. Click on it, and a box will pop up. Switch to the appropriate type of flour, and put ten pieces of barley/wheat for one sack of flour, twenty pieces for two bags, and so on. Remember, you cannot mix barley and wheat into one sack of flour.