Dave Weinberger summed it up:
If it's Wednesday and you just found an exciting shell, you'll have to wait a week to tell the real world Shell Club about it. But, if you were on a mailing list, you'd send out an email on Wednesday afternoon. People would read it whenever they wanted. People would respond when they wanted. The conversation isn't confined just to Tuesday afternoon. It's always there, going on with you or without you. You can jump in when you want.