A common shortcut when using a computer is, instead of typing something from scratch, to copy some similar looking text, paste it in the new location, and edit it.
A common error (especially while programming or while labeling a document/spreadsheet) is to copy some text from Place A and paste it
in Place B intending to edit it there, but forgetting to do so.
For example, if you've typed this line of code:
row(1) = row(1) + column(1)
and now you want to type
row(2) = row(2) + column(2),
you might be tempted to start by copying and pasting the first line, and then manually changing the 1's into 2's.
But in this world of constant distractions (emails, phone calls, constant new ideas; general forgetfulness), it's likely that you will perform the copy-paste but then never finish typing the 2's over the 1's.
This is a difficult error to detect when debugging, since the code "looks right", but one which undoubtedly makes the user feel stupid after he/she discovers the reason.
I recently saw a spreadsheet where a label had obviously been copied from one column to another but where the writer had failed to edit the pasted text and ended up having two identically labeled columns.
The "Paste And Change" option (shortcut: Ctrl-H) would end this problem. When "Paste And Change" is selected instead of simple Paste, the copied text is pasted but also highlighted in a special color (dependent on the application, but also changeable by the user).
The highlighting will remind the user that this text still needs to be modified, and that you didn't just copy it to duplicate it but also to change it slightly.
At the user's discretion, the application can remind the user before quitting that there are still areas which need to be edited; do you really want to quit?