Second, standing puts you in a physically dominant position. This sounds politically incorrect, but in this context it isn't. The audience needs your help to maintain their attention. They want you to be in charge. This is one of the most important principles of public speaking.
People are visual creatures.
At a minimum, have an outline of your talk on overhead transparencies. Some people seem to think they're giving everything away by showing people what they're going to say before they've said it. At the same time, it helps them understand the structure of your thinking. Talk outlines should be extremely concise and visually uncluttered. 12-15 lines of text per transparency is plenty.