There are many - really many - possible usages for a reasonably powerful processor that can 
establish connection with both peripherials and PC using more than one protocol and can be programmed
using more than one language. So I will only list what I consider most useful.
1. It can be used anywhere the Arduino can. 
2. It can run a simple Web server in your local (between devices that use WiFi served with your router)
network.
3. It can be used as a router, though it is not recommended due to security reasons.
4. It can use Sockets and Server Side Events: both allow "pushing" data from server to
client, which means that you don't have to keep sending request to server by timer.
5. It can ger to the "outer" Internet, for example, to get exact time.
6. It even has a slot for SD card and can run a file system.