Manually Switching Signature
The user can quickly switch the signature on a mail-by-mail basis if you have deployed multiple signatures.
It could be that you have distributed several languages for the sales department.
E.g. Then if an English user is writing a mail to a German the user can right-click the default signature and get a small drop-down to choose from other signatures.
In this example there is only one signature, but you can see the menu that will show up.