Almost everything about it needs to be optional because sometimes USB is used to charge some cheap battery powered thing and sometimes it’s used to make a backup of a harddrive and sometimes it’s charging my laptop with enough power for it to be rendering video but still have a net charge increase to the battery while also providing Ethernet, video output, and keyboard/mouse input over the same one port.

EDIT to make it more clear why the variability of USB standards is what it is, compare a modern laptop to one from 10 years ago.

The older laptop has:

• for video, an HDMI port (or the less common mini HDMI port), and perhaps a mini DP port
• an Ethernet port
• a charging plug
• possibly some FireWire ports (may or may not be the same as the mini DP port)
• USB A ports for keyboard/mouse and other random devices

The newer laptop has:

• USBC ports that can do all of the above

The perhiperals, however, don’t support all of the features. They only support the features they actually use. As long as the laptop supports all of the optional features, you don’t need to worry about it.

The is especially helpful for less technical users who may not want to know what the difference between HDMI and DisplayPort is. With a fully USBC based laptop and USBC perhipals you can just plug it in and it will work.

Of course this is all dependent on the laptop implementing all of the extra features, which is still only really true of more expensive laptops.