1.The speaker needs to separate himself from the listeners. An introduction helps with the transition from being in the group to stepping in front.
2. The listeners will be more perceptive and appreciative if they understand what the speaker is undertaking.
3. The audience should know the special reason the speaker has for speaking about the chosen topic, and any experience or expertise the speaker has in the subject area. A good introduction makes it clear that the speaker speaks from special knowledge, experience or authority.
4. An introduction "builds a bridge" from where the thoughts of the group are to where the speaker wants them to be.