1. it is cheaper to implements. For example, if you develop application with .NET you need to have clients that install windows. but in web application, once you develop, any client can access your web apps with any platform.
2. web app development are simple (once you know about css javascript and framework) it can make your life easier.
3. No virus and easy to deploy/install for client.
4. Only one copy of a program will ever need to be updated.
5. 99% of the code is platform independant.
6. Less chance of finding restrictions that the clients computer may have imposed
7. Accessible from wherever your users are
8. Cross platform compatibility in most cases (i.e., Windows, Mac, Linux, etc.) because they operate within a web browser window.
9. No installation required for the users
10. You can easily measure what your users are doing
11. You can provide easy collaboration between your users if necessary, as all data are centralized
12. Don't have to enforce version check in client machine.
13. Support and maintenance are easier.
14. Web applications integrate easily into other server-side web procedures, such as email and searching.