A2R is a switch with the capability to share one amplifier between two radios.
If you have a Solid State amplifier with fast band switching, it can upgrade your amplifier to become a real SO2R amplifier.
If you have tube amplifier, you can share one amplifier on a single band, between RUN and In-Band operators.
It can be easily connected to two standard radios, one for RUN and one for In-Band operator.
If you have a FlexRadio than it is even simpler – and radio can handle both operators, connected to FlexRadio from maestro, or SmartSDR
To put it simply – if you have a need to use two radios on one amplifier it will enable you to have manual selection of which radio will take the amplifier and it will make your wiring easier and usage more comfortable.