If replaces the variable’s value with the value of the variable named by the value of the variable in question. Basically, it is an indirection, allowing you to read the value of a variable whose name is in a variable.
It is the equivalent of
Set variable Name to %Variable%(%Variable%Name%)%
which does not work and which you can’t do easily.
It is equivalent to
Set variable Name to %%Variable%%%Variable%Name%%%
Filter Variable "Name" Process Tokens