Stormy Peters: What should the GNOME Foundation accomplish in 2010?

Pressing Ctrl-C or Ctrl-X fills the "clipboard". Pressing Shift-Insert or
Ctrl-V pastes the "clipboard".

If you question is "why don't we merge the clipboard and the selection
buffer", a number of old buggy applications did just that. Then you
couldn't rely on the contents of the clipboard buffer being unchanged until
you press Ctrl-V. You could carefully select some text, click on another
window accidentally select some text, and boom! the text you carefully
selected is instantly blown away. Keeping the selection and clipboard
separate is also less confusing to windows users as they can use Ctrl-C and
Ctrl-V etc. exactly the way they use them on windows and ingore the
selection buffer completely.

Keeping the selection buffer separate has another advantage: you can store
two pieces of text at the same time. So you could copy one section of text,
select another. Then you when you switch to the other window you can paste
the selection buffer and the clipboard without having to switch back to the
original window.