A way to insert X11's "clipboard" buffer (i.e text that is currently selected text).
I agree with GP: I find the two separate copy-paste methods very useful and wouldn't want to see them merged. Users who aren't aware of one of the aren't affected, and those who are get an extra buffer to use.
Clicking with the middle mouse button - nowadays, it's typically a clickable scroll-wheel, but it used to be a plain button between left and right mouse buttons back in the day.
After mice had mostly standardized on two buttons but scroll wheels didn't exist yet, it was common to map a simultaneous click of both mouse buttons to simulate a middle click.
