I’ve just picked up a rather nice Joystick nd was looking a support pack to gain a little more runtime. I may being a bit thick but what do they do that a decent USB power pack doesn’t?

Nothing – as long as you buy a power pack that can output a decent current

You also might not want a powerpack sitting on your lid charging your light on the go, and also won’t be water resistant

Mainly down to having the right connector and curly cable which means it’s easier to fit, particularly if strapping to your helmet. It’s also more weatherproof. Some of the USB powerpacks aren’t very good with the vibrations of being fitted to a bike – vibrates the components loose

Waterproof.
Same charger/cable.

I got one with my Equinox when Wiggle were selling the pack for less than an Equinox on its own.

I got smudge at mtb batteries to make me one. Less polished finish but has been reliable in all sorts of weather.

I’ve mailed Smudge but haven’t heard back – I could’t find them on his site

I don’t think he does list them as there are a few options. I went for a triple cell with 1.2m of cable so my diablo can keep pace with my six pack and pop the battery in my jersey pocket.