It depends on the situation.
A 20A DP switch must only supply one single socket if it is on a circuit with a rating higher than 20A
13A FCUs can be used to supply more, but you are obviously limited to 13A by the fuse.
Also the fuse in the FCU may not provide discrimination with the fuse in the appliance plug.
It is up to the designer to decide what sort of switch is the most appropriate for that particular installation.
You wouldn't have more than one appliance on a switch.Why can you supply only one item from a 20amp DP switch, and more with a 13 amp FCU?
it is easiest to put appliances under the worktop if there is a 13A socket to plug them into (they come with a 13A plug)
so to avoid duplicating fuses, it is convenient to have a 20A DP switch above the worktop controlling an unswitched single 13A socket below.
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