If you could pick just one advantage of the Internet over every other popular medium out there, that advantage would be its interactivity. We can communicate with users, customize their content, and most importantly, make sure that they can only see what they're supposed to see.
ASP.NET originally shipped with a fairly slick mechanism for identifying users, and it saved programmers a ton of work. Microsoft has taken this to a new level in ASP.NET v2.0 with the addition of the Membership programming interface. Now we can whip up a mechanism to register and manage users with little to no code in a matter of minutes.