The 1:1 FMU is used to counteract the rising manifold
pressure.  Think of it this way; if you have 10PSI of boost
inside the manifold and 50psi of fuel pressure, it's the same
as having no boost in the manifold and 40psi of fuel pressure.
 The injectors have to have more pressure behind them to keep
the air from 'pushing back' against the incoming fuel.

As far as your map is concerned, you shouldn't ever notice it.
 But if you're running significant boost, you'll certainly
notice it if you don't have one later on.