The use-case that prompted this suggestion is that I have the Steam app on my phone, and on my desktop PC. I'm using a normal LastPass Password (username + password) to sign in on my phone, and an Application to allow me do use the Ctrl + Alt + F autofill on my PC.
The problem is, to the best of my knowledge, that I have to duplicate the credentials for the two secrets, which adds a bit more overhead, especially when the time comes to update it.
My suggestion is an optional drop down-menu in the "Add/Edit Application" pop-ups that allows us to choose a pre-defined set of credentials to use. If no relevant credentials are present, we can continue to use the Username- and Password-fields like normal.