What is the actual purpose of the USB port in the PSTV?

The PSTV (also called PS Vita TV) has a USB port as part of the unit, but what is this port actually used for?

As far as I know, it can't be used for data transfers and/or peripherals so it seems kind of useless.

Officially, it seems like it could have been used to connect controllers, according to diagrams from Sony:

The same source states:

To play games on your system, you must have a USB cable. USB cables differ depending on the controller you want to connect. For details on USB cables, contact customer support in your country or region.

Unofficially, it could also be used to get additional storage through some homebrew software, as discussed in this video