Well one use really.
We use a cheap Motorola Moto E for navigation, running Viewranger in the UK and OSMAnd abroad, but its battery does not last the day so we have a battery pack to keep it going. Our new shiny, that I may have mentioned elsewhere, has a larger diameter stoker stem requiring a 30mm clamp. Making use of an old Cateye seatpost fitting and a borked rear lamp casing, thanks Biggsy, we can now use one side of the battery with the new shiny and the other side usung a different mounting clamp on our Dawes.