Why is it necessary to add oil to bacon and onion when cooking in a frying pan?

In general, you do not put oil in the pan when frying bacon. Bacon is fatty and the pan will gain a layer of liquid fat on the bottom when the bacon heats. In fact, if the bacon is very fatty and the fat is not drained from the pan partway through frying, the bacon might not become crisp.

If you are frying healthy alternatives to bacon, the pseudo-bacon might not have enough fat content for frying, so you might need oil, but check the instructions on the container in which it came. (I tried two versions of soy bacon many years ago, and one suggested putting oil in the pan.)