1) Copy some cells. Type something into another cell before pasting it. Your copy buffer is gone, you can't paste what you copied.
2) You copied some cells, pasted them. You arrow over a few cells to inspect a formula and then hit enter: You just pasted over the surrounding area with whatever you've copied previously.
3) You copied some cells, you pasted them somewhere. Now you want to insert a new column, but when you right click the option is missing, replaced with "insert copied cells". The clear the copy buffer you go to random empty cell, type a space to clear the buffer (see #1 above) and now you can insert a new column.
On the plus side though, copy/pasting formatting is pretty useful, so is transpose.