So that you can run several appliances off the same power point. Maybe you don't call those things adapters? There are several types of adapter.
The gizmos that allow you to plug US equipment into a UK, or other system, power point are adapters and maybe some of them adjust for voltage difference as well as the difference in plug pattern (round pin, square pin, two pin, three pin etc.). Normally, I believe, they just allow for the different pin shape and pattern and if you come from an area with a 110V supply then you should only use them with equipment that has a voltage change switch on it (e.g.: from 110V to 240V)otherwise you may burn out your equipment.
The adapters that allow you to plug UK equipment into US power points don't have transformers, that just means that a travel kettle, or set of electric curlers, takes longer to heat up if the voltage is lower unless of course it has a voltage change switch.
The other type of adapter fits into a power point and has two or three or more outlets thereby allowing you to plug more than one appliance into the same wall socket outlet. With these it is important not to overload the power point.
