Web applications are software developed using typical web technologies (HTML, PHP, etc.) hosted on a web server and accessible through a web browser. Usually web applications are online applications, that is they are public on the Internet and you can access to them anywhere; still it's possible implement offline web applications, these are applications that are hosted within the LAN network (intranet).
The main benefit of web applications is definitely remote access: they are already a system ready for the remote access: for example one can implements a software that allows agents to access and insert orders or offers while they are at the customer's.
Another benefit is the accessibility by several devices: you can use the application on a desktop computer as on a tablet or a smartphone, without the need to develop additional software for such devices.