The reason you see these in Admiral and the like is basically to bend the B3 licensing slightly - they can have 'Up to a maximum of 20% of the total number of machines available for use on the premises'
So they shove these in between the B3 to up their machine count, and double up as a coffee holder!
Just had a go on this and it seems very hard to lose more than £20 overall no matter how much you put through it, is there a RTP attached somewhere?