I found a Gardena connector under a hanging basket at the local pub - it looked
like they had had the baskets "re-plumbed".

The item is Gardena 8337-20 .

I have never used same - I usually use more basic straight through connectors
- and have never had any problems with them eg Hozelock Tube Adapter Pack -
2778 (I know they are for screwing in to bigger pipes for pipe reduction).

My question: why on earth do Gardena go to the trouble of putting the metallic
(I assume) locking clip: do people who use them find them any good or
necessary?