Those couplers have been common in the U.K. for many years on appliances such as lawn mowers, hedge trimmers, and so on.
Another reason for using them in preference to BS1363 extensions is the rating of the cord. Far too many people will try to use any BS1363-equipped cord to run anything they can plug into it, including a 3kW fan heater! Although smaller gauge BS1363 extensions will come equipped with a 3 or 5A fuse in its plug, we all know how often that gets replaced with a 13A type.
At least the use of these "dedicated" extension discourages such abuse with high-power appliances.
I've noticed that quite a lot of the latest garden equipment sold in Britain comes with latching IEC-style connectors these days. In some cases, there is no cord attached to the appliance at all -- Just a molded IEC plug in the casing for the attachment of the cord.
