This makes deployment much easier. The user doesn't have to install anything, and the application silently upgrades on the next request (if a new version is available). This also makes server development easier since it's mostly not more than one client version that is used.
Built-in network client
It helps a lot that the web browsers handle all network connections for you. In example when you want to post messages to the server, you have to do almost nothing to handle the network connection. Compare this to using Apache HTTP Client, specially when you want to use a secure connection via HTTPS. The new Websockets will make this even more powerful.
Easier to make layout
Easier to make asynchronous programming