OK ,
in WDM every step needs specific power for example the MUX board like M40 needs all input channel to be in the same power or at least have small power difference between channels .
in Amplifiers each type of amplifiers has a specific range of input power ,the input power should not be bigger or smaller than that range , also in that power vlaue range there is an ideal value that gives u best signal to noise ratio.
The amplifier receives signal from :
1- Mux board .
2- after another amplifier board .
3- direct from line .
and in most cases the coming signal comes with power higher than the range of the amplifier input power and need to be adjusted before getting in the amplifier , so usually we input the signal to a variable attenuator before getting it in the optical amplifier .
the VI/VO gives u a builtin variable attenuator and this is a good point.