Baked a vegan cake this weekend and it seemed to be a step in all the recipes I found.
Maybe to replace buttermilk.
Don't know for sure but....maybe to make soy cheese?
This milk + vinegar technique is used widely, not just in vegan cooking. It gives a simple alternative to cultured milk (or milk alternative) products, such as buttermilk.
Making a soy cheese is a much more complex project.
The acid breaks down the protein, making it more amenable to being used in baked goods without their being heavy and tough. Too, it reacts so nicely with baking soda, again helping ensure a lighter product.
Apple cider vinegar is considered a healthier/higher quality alternative to other vinegars by some people. The curdled nondairy milk usage is common in vegan baking to introduce an acid to react with leavening agents, like Cynthia mentioned. This helps vegan baked goods be lighter, a balance which is often disturbed when substituting eggs in a traditionally formulated recipe.