Users sometimes have multiple addresses for the same account (called "aliases"), but want to send emails specifically from one address and/or have different signatures.
If you reply to an email, GMail will automatically send the email from the account the original email was sent to.
The short video linked below is more specifically about signatures (and about marketing a signature generator), but it does show how to set up multiple signatures as well as there's a quick spot at the end where they show changing the "from address" to the non-default address.