To preach about something when you have good experience and knowledge of that subject and you would like to spread this knowledge across a wider audience in order to make them aware, it's a good deed of sharing and caring.
The person considered to be a good preacher himself understands the subject better and later understands the audience better and then establishes a connection that will help them to impart that knowledge.
The challenge here would be that each one has a different level of understanding and intellectual capacity so what may sound simple to me is not necessarily simple for another person. In such a scenario the person needs to first understand the audience and accordingly convey what he is trying to say, because a school-going kid may not be able to match with a Ph.D. student.
The whole purpose is to serve a wider audience in such a way that everyone gets benefits and that's when you can say you are a good preacher.