There are two main reasons:
First, whether there is an accurate amount of water, because of the different environmental conditions or the nature of raw materials, for example, the same bag of flour may absorb water in wet weather, leading to a decline in eating capacity. When mixing dough, water will be added to make it mix evenly. There is also an industry term called "rejuvenation water", which is to stir the dough a little harder first, and then add a small amount of water, so that the dough will be smoother, more elastic and more colorful.
Then, most importantly, flour is not a single substance. The main ingredients of flour are starch, fat, protein, minerals (called ash in the past) and so on. Only protein will form gluten after swelling by water absorption, but the proportion of starch (85%) is much higher than that of protein (about 12% ~ 14%), and its water absorption is also stronger than that of protein, just absorbing water. If water is poured in at one time, even if the amount of water is just right, the water gathered in one place will separate the flour, and the starch will preferentially absorb water, expand and gelatinize, so the dough is sticky and wet. However, protein is actually in a state of water shortage, and there will be problems in the formation and expansion of gluten network.
When making bread and noodles, there is a basic common sense that if it is found to be too dry, you can add liquid (water, milk, etc.). ), but if it is too wet, you can't add dry powder, especially when the stirring time is long, the gluten is initially formed and the butter has been added. Adding dry powder will not help, and it will destroy the uneven gluten and affect the fermentation. The surface of the finished product will crack, and there will be raw flour and internal pulp.