When holding a mouse in your right hand, the left mouse button is the "default" button to click. So it finds itself used to click buttons, select from menus, move input focus, drag and drop, and any other simple task.

The left click is (for right-handed users) the only click that can be doubled or tripled.

Additionally, in X11's standard Athena (aka Xt), left click performs these actions:
  • In the text widget, a left click (followed by a drag) is used to select. A single left click is used to mark a region composed of characters. A double left click is used to mark a region composed of words. And a triple left click is used to mark a region of lines. Extending the region with a right click continues to use whichever category of the 3 was used to create the region.
  • In the scrollbar, a left click scrolls forward. It brings the row of the window alongside the cursor to the top row. So the further down you're located, the more it scrolls; the slider's location is irrelevant to this action. Contrary to popular belief, this is far more convenient than other, flashier but badly designed scrollbars. This action is directly inverse to the action of a right click in the scrollbar at the same location.