There is nothing wrong with IIS and you should use it if it's possible. However, home users and small business owners usually don't have IIS installed on their desktops, and they often lack IT skills, or even a media, from which IIS could be installed. If you wrote or have an idea for a home or small office mass-market web application, no IIS on your prospects computers will be a major problem for you.

Enter UltiDev Cassini Web Server.
With its tiny footprint it won't add up much to your distribution package. Also, Cassini is exceptionally developer-friendly with its pre-packaged redistributable components, its debugging capabilities, Cassini Explorer application management console and a .NET assebly for registering/unregistering applications with Cassini server.

If your thought about writing that killer ASP.NET application for home users or office desktops, it's time to do it!