The saved labels are great for organising emails into pre-conceived categories.
But often I just want to put a unique label on an email so I can easily identify what it's for.
e.g. I have a label "Bookings". I slap that on all my bookings and tickets: flights, hotels, events, restaurants.
But it would be really handy to be able to add a unique label (not to be saved) to each one so that I don't have to open various emails to find the ticket I'm looking for.