They are used as either a binder or protein source or both.
For example, in making some breads, either milk or eggs are added. Many of the recipes for my breadmaker called for milk, and when I added store bought almond milk, or omitted it as an ingredient, I'd have problems. I didn't understand that I could just add a vegan source of extra protein (such as 1/4) of the flour weight as white flour (for protein), or cooked/mashed beans for extra protein.
For binders, or to add fat to a recipe here are some:
13 best vegan egg substitutes for baking and cooking
