What is the middle joiner in this pic used for, the brass looking one with the 6mm barb?

I got a couple of these packs in my recent order from Alex and wasnt sure what these would be used for?
Ta muchly.
Matt
The kit pictured is a microbore/minibore fitting kit for a metal hose reel.
The brass connector you are asking about will screw into the internal hose fitting on one of our heavy duty metal hose reels as shown below. It screws into the inside of the hose barb.

The kit you have picture will connect a reel of Microbore/Minibore to a metal hose reel and then terminate it in an EZ snap end stop.
If you are using a plastic (Hozelock) reel then you need the Microbore/Minibore fitting kit for plastic hose reels, same price just come with an adapter and section of 1/2" hose to connect to the 1/2" fitting on the inside of plastic hose reels.
I hope that this makes sense. If you have bought the wrong kind of fitting kit for your hose reel then I'm sure that we can exchange them for you.