There's not much else to say that hasn't already been said.
The purpose of the USB Host interface is to allow you to put a thumbdrive in the unit and update the firmware, save and load instrument settings etc. It can also be used to connect the optional USB<->GPIB adapter.
The purpose of the USB Device, RS232, Ethernet and GPIB interfaces are to allow you to programmatically control the instrument. Electrically and physically the interfaces are different (obviously) but the commands you use to control the instrument are the same, whichever interface you choose to use.
The purpose of the digital trigger input(s) is to allow you to remotely control the state of the output(s) with a digital signal.
The purpose of the digital trigger output(s) is to allow the instrument to "put out" a digital signal when a certain event occurs (for example when an output goes into CC mode).
ALL this information IS in the manual and in the programming guide.
When it comes to programming/controlling the instrument remotely over USB, GPIB, RS232 or Ethernet you REALLY need to do a bit of reading on the subjects of VISA and SCPI to understand what it's all about.
Here's the
manual.
Here's the
programming guide.