When you insert or delete a row or column you are adding a new row or column to the spreadsheet. For example, if you insert a column at the beginning of your spreadsheet, everything that was in column A is moved to column B. Everything that was in column B is moved to C, and so on.
When you copy and paste a you are replacing the contents of existing cells with the content of the cells you just copied. No new space is created in your table.