There are a thousand and one reasons to teach students how to create a website. Here are a few:
Student Portfolio - Students create one site and use it throughout their academic lives to add to it- essays, projects, photos of projects, reflections, etc.
College Entrance Portfolio - Knowing how to design a website is a great way for students to easily create a portfolio to submit to universities.
Student Project Site - Students can maintain a website dedicated to a class project such as Genius Hour, 20-Time, etc.
Group Project Site - Students can work in groups to complete a complex project. The key to being 'fair' in this group activity is to assign each group member a specific research webpage in that website that only that group member will be graded on. I would suggest creating the sites yourself and then sharing the sites with each group individually- this way you have ownership over the site.
Hobby Site - Once students learn how to create websites, it is easy (and addictive) to create more sites. If they do this outside of a school assignment, encourage them to use their personal accounts to do so.
There are some great reasons for teachers to create and maintain a website as well:
Teacher Class Website - Create a class website for students and parents to refer to.
Teacher EduHobby Site - Do you do something in class that you'd like to share with the world? Create a website dedicated to your special class project and teach others how to implement in their classes.
Hobby Site - You'll find that once you create one site, you'll want to create a bunch of sites. Why not? Use your personal Google account to create your own website dedicated to your hobbies.
Class Project Site - Utlizing the group project sites in the student section above- link each group site to a class project site for easy access. You can add resources and project requirements to this class site.
PLC/Department Site - Create a robust and informative website for your PLC or department within your school.
Check out this quick tutorial on how to use Google Sites. Feel free to share with others and your own students.