What job is the fat doing when I prepare bread dough, and what to expect if I use the wrong amount? 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.