Each of the three ports on the back of the hub have the ability to provide up to 15W of power to connected devices, and this is typically used to power an external device (such as a Thunderbolt or USB-C storage drive). 

However, this power can also be used to charge devices that have an internal battery, for example a mobile phone or a tablet.

An additional useful component of this feature is that the charging capability can be used even when the hub is not connected to your PC.  

To put that another way, when your PC is not connected to the hub, the hub can be used as a charging station for up to three different devices.