Peter Mead takes on some of what he perceives as myths of outlines and understands the benefits of outlining to be mainly for the speaker. I think that is good as far as it goes but it fails to appreciate the tangible benefits for the audience. Before proceeding to my response, read his post here.
I believe that a well crafted outline not only helps the speaker, but serves at least three significant purposes for the hearers. First, it reinforces the idea that there is a particular logic in the case of more didactic texts and a particular flow in the case of narrative texts. That is, outlines remind the listener that an argument or story is going somewhere. Second and related to the first, a good outline helps the listener to more easily anticipate where the preacher is going and to remember where the preacher has been. If I know a speaker’s outline, then I can more easily anticipate where he is going and as the message progresses, to frame where he is going in light of where he has already been. This can be invaluable in properly understanding and appreciating the material in any given section. I can recall many occasions as a hearer thinking, “Where is he going with this?” While creating a sense of anticipation can be a good thing, a listener who has to expend too much effort trying to figure out where a message is going is often going to miss important parts of the message itself. Third, an effective outline is a helpful teaching device in and of itself. A speaker always does one of two things. He communicates content (directly) and he communicates method (often indirectly). My first two points have addressed mainly the former, this last point addresses the latter. Outlining is a not only a tool in speaking but it is also a methodology for reading and studying. When a good outline is utilized, the audience implicitly sees the advantages of the method, thereby adopting the method as they might study the Bible.
One final point, Peter makes part of critique of outlines based on what audiences usually remember, but if memorability were the sole criterion for how we craft our sermons then I fear that we would fill it with illustrations and jokes. The fact of the matter is, many people remember very little of anything that we say after a few days. This is why my last point above is really important. I don’t care if the congregation remembers my outline, but I do care if I help to teach them implicitly the value of outlining and even to some degree, how to do it.