Many people using the web do not have e-mail accounts. Some of them are using a computer that belongs to a friend or relative, and an e-mail address is of no use to them. A web page is available 24 hours a day, while an e-mail address is only good when it's being checked regularly.
People sometimes go on holiday, take business trips, or become ill.
Such occurrences can last days or weeks, while someone else is awaiting a response. In our mobile society people have a tendency to move frequently, sometimes with little or no advance notice. The move may involve a web page, an e-mail address, a street address, or some combination of the three. Usually, though, all three will not change at once, and you can still be contacted. It's also very helpful if a research index allows those visiting it to quickly determine who are their closest cousins.