My most often-used Calibre functions are, in order:
1. Add book (but I do this by dragging a book to the Calibre window. I didn't even know that there was a button for it
2. Edit metadata - to clean up the metadata, add series information, or download a cover.
3. Send to device. (But now I do this by the very useful shortcut of simply dragging the book onto the "Device" button on the toolbar.)
4. Convert book (mainly used when I've changed the cover, to make the new cover "stick".)
5. Remove matching book from device (after I've read it.)
And that's really all the Calibre functionality that I use.