A principle is a moral heuristic so you don't have to scan to root of your moral argument tree on every moral question. As a heuristic, it attempts to compress the moral space down and will naturally have exceptions.

As a compressed form of your moral judgment, it is also useful for you to quickly communicate your position on the moral space, and it is up to participants in social interactions to decide whether or not someone is good at this moral judgment compression.

For instance, I have a principle not to lie in general. Most social participants know that this does not constrain me from lying in specific situations. After repeated interactions they can tell whether this principle of mine is held in a manner that's useful to them.

The same principle also allows me to quickly judge situations that require me to lie. Is it okay for me to tell this person that their house is painted poorly? Well, I have this anti-lie principle so that pulls me toward "yes". I might have other principles that pull me in other directions.

Of course you can treat principles as immutable moral rods. This presumably has some value to you. In general, people do not act in this manner, however.