I've been baking bread for years and no longer really measure the ingredients. But I'm no expert on the science of what I'm doing. I add fat (lard) because I've always added fat.   What job does the fat do? And if I have long been using too little or too much fat how would this affect my loaf?   And then, writing this up, I'm also wondering about yeast. I guess if I use too little yeast I am going to get a loaf which rises insufficiently. But what would be the consequences of using too much yeast? 
Amongst other things, fats help moderate/impede gluten development, by not allowing water to activate the proteins.  Too much yeast and your dough will be flabby and over-risen. 