I use middle click to paste all time and it's one of the reasons I bought only laptops with 3 hardware buttons on the touchpad so far. How does middle click work with the kind of touchpad on the XPS?
Buttonless touch pads can be configured to support different actions for two/three finger click [1]. I use Gnome where the default is right click on a two finger tap and middle click on a three finger tap. [2]
They most probably know about the keyboard shortcut to paste.
Linux' "Select to copy + middle-click to paste" just saves a lot of time, and has the extra bonus of being a clipboard parallel to the one accessible via traditional copy-paste shortcuts and GUI menus.....
Many people select text using the mouse, then reposition the cursor using the mouse. It's convenient to then insert the selected text using the mouse — the entire operation is done without touching the keyboard.