Look at a chart on the Maillard reaction*
Note: 1) using leaven and olive oil makes dry firm/soft bread, 2) oil probably does not contain water, 3) fat appears to contain water, 4) the word fat used in this blog post may only refer to butter
Note: 1) another combination is oil and butter (flavor and moisture) (make biscuits or pizza crust with oil and butter), 2) find the number of combinations without repetition (leavened, not leavened, oil, fat)
When baking, the butter in puff pastry melts and releases moisture. What is the word for fat and flour cooking together while baking? Does the word frying work? Is steaming a better word? If I find a word, I will put the word in this blog post.
People can describe what heat does to the outside of something made with puff pastry using the Maillard reaction. To understand baking better, someone should start by making unleavened bread. After making unleavened bread, a person should consider making leavened bread without fat. Then a person should make leavened bread with fat. Imagining that all of these different types of bread have been baked, I am suggesting that they are all similar because the outside of the bread is browned. Each type of bread is different because bread made without leaven is hard and brittle, bread made with leaven is soft, and bread made with leaven and fat is soft and flaky or moist (humid). The missing combination is unleavened bread made with fat. I do not have the experience to imagine the appearance of buttery matzo.
What is the effect of using too much fat? I do not know. I have made matzo, French bread, and biscuits. Matzo is unleavened bread, French bread contains leaven and no fat, and biscuits use leaven and fat. Each type is browned on the outside. Matzo is hard, dry and brittle. Leavened bread without fat is soft and dry. Leavened bread with fat is soft and flaky or moist (humid). I am able to observe that the fat makes the bread moist. The remaining oil from the fat must change the texture and flavor of the bread.
Without doing science, the answer to the question asking what is the effect from using too much fat when making bread must be that the bread becomes more moist by adding fat until someone adds enough fat that the flour cannot be used to form bread dough. The extreme example is making bread dough that melts similarly to a cookie. Cookies contain great amounts of sugar. Maybe an unsweetened cookie is what someone makes when too much fat is added to bread dough.
A conceptual explanation of the Maillard reaction can describe what occurs to the outside of bread when someone bakes bread. A scientific explanation includes knowledge from chemistry. Following the link above may provide graphics that explain how the shapes of the molecules change based on specific cooking and baking circumstances. Different fats and temperatures change the Maillard reaction. Conceptually speaking, the sugar melts, browns, or burns. Caramel is browned sugar. When the sugar in food is heated, the sugar can brown. The sugar can become caramel. Caramel is candy. Caramelizing something changes sugars in food to become candy. Browned biscuits are flavored with caramelized sugars. A general rule for cooking is that more heat makes sweeter food. Unless the food burns. Burned food supposedly lacks flavor. Burned food may only have texture.
* Maillard Reaction. Digital Images From A Video. Top Chef Masters, Season 3: Blinded Me With Science. From: YouTube, https://youtu.be/Q41XkYXJ9xM (accessed December 10, 2016).