Great question! The biggest reason is as instructors, we want to provide our students with solutions to threats. I think most instructors have good intentions. But if we try to have a solution for everything, we will come up with answers based on our training and experience to do it. In many cases lacking the experience, and knowledge to give those answers. Why? Because we are often drawing from the same well.
In other words, to put it in a martial arts example, if you practice kickboxing and what to learn grappling, you don't get their by studying kickboxing at a deeper level. No you go to an expert to learn what you don't know about grappling. If you want to teach self-defense at a higher level, you don't get their by becoming the best combatives guy in town. You have to discover nuances you are previously unaware of.
By being aware of the nuances, we can begin to broaden our understanding of many topics and skill sets we were previously unaware even existed.
This thread is NOT intended to be a "stay in your lane" speech. The fact is the student or client is going to reach out to you as a martial arts instructor in many cases, regardless if it's the right context. But if we are self-aware, we can better begin to understand what type of skill set they best need. We can grow in ways we didn't know existed, to help them. We can teach better classes. On the other hand sometimes, this may simply mean saying, "You don't need martial arts instruction, you need...."