The first time you use the Web Conference Centre, you will have to join the Math151_d1 Web Conference. Only students currently registered in Math 151 will be allowed to join.
Why use a Web Conference instead of e-mail? The main reasons are those of storage and organization. All messages posted will be kept in a central location, stored by subject thread, where you can easily view the progression of a discussion. E-mail, on the other hand, fills the mailbox of everybody on the mailing list, even those who have no interest in it. Or you might think you have no interest in it, delete the message, and then realize later it was important but the message is gone.
The downside of a Web Conference is that a person has to actively check the conference site and many people just don't bother. Conversely, almost everybody reads their e-mail. Therefore, it is probably best to use e-mail if you have something urgent and important to say in a short message. If it's discussion which only a minority would be interested in, or a long-winded posting, then it's better to use the Web Conference.