Lecturing is a skill that is in short supply. Unfortunately, university policies don't prioritize lecturing when hiring professors. Most professors are offered positions at universities because of their talent as researchers; however, good research does not imply competent teaching. This may not be true in all faculties, but it is certainly the case in science, engineering, and medicine.
University policy often dictates that professors must spend a certain number of hours lecturing students. I do certainly see the validity in the policy; it would be ridiculous if you had a world renown professor working at a university and none of the students were given a chance to learn from him/her. However, forcing professors to lecture when they are astonishingly bad at it is a recipe for disaster.
In medicine, we have had many lecturers, some good, some bad, some terrible. There are often many common flaws that are ruinous to any lecture and one of them is not starting off the presentation with a slide stating the lecture's objectives.
We need to know what we are supposed to learn from the talk before you start lecturing. If we don't, we can't filter the vital information from the extraneous or esoteric. If we can't filter out the unnecessary, we are overwhelmed with the overload of information and it all becomes white noise. Remember, in medicine, we have a lot of information that we need to absorb...if you don't help us prioritize, we will resent you for it and not learn properly.
It really isn't hard, just give us bullet points on the information in your lecture that you think is absolutely essential for us to retain. Then, even if you are disorganized and jumping around from topic to topic (which you shouldn't be doing!), we know when we can relax and when we absolutely have to take the time to note specific details you may have forgot to put in our note package.