Symposium is like a special issue of a journal. Part of audience have come just to see what speakers look like, some want to learn what all the fuss is about, some should be on the panel rather than in the audience, some have come to give speakers moral support, some want to meet speaker afterward, and some are just tapped out on the convention and want a place to rest. Safest assumption is that participants are thinking what they want to say and are not listening to others, at least not until they give their talk. Thus, the 1 st speaker will have heard more of what is said than the 2 nd , etc. The first speaker often has been at a different symposium than the last speaker.
First speaker is more of an ally for discussant.
Because participants are preoccupied with talking rather than listening, they will not have picked up on many connections between the papers. When the discussant mentions some obvious ones, that will usually be the first time the participants have thought of them. Also, because listening may be very uneven, it is a huge help if discussant says what the core idea is in each paper. This is tough and risky. Participants and the audience as well want the discussant to give a context that makes sense of the papers, be enthusiastic about the work, and improve it by extending it and by making constructive replacements of poorer methods and arguments with better ones.