It is actually explained very nicely on THS's website!
Pressure drop
How does reducing pressure drop help?
On the 2.0TFSI engine the boost pressure is measured on the up-pipe after the intercooler. However the boost pressure here, or at the intake manifold will be less than that at the turbo outlet. This is because of the restriction of the intercooler and pipe work.
Example restrictive core
Intake manifold pressure: 25 psi
Restriction / Pressure drop: 6 psi
Turbo outlet the pressure: 31 psi
Reducing the pressure drop by means of a more efficient intercooler and pipe work requires the turbo to produce less boost pressure at its outlet whilst still maintaining the required boost pressure at the intake manifold.
Example free flow core
Intake manifold pressure: 25 psi
Restriction / Pressure drop: 3 psi
Turbo outlet the pressure: 28 psi
This has 2 distinct advantages:
Compressing air causes it to increase in temperature, the less it is compressed the colder it remains.
An intercooler which offers good cooling but with a high pressure drop due to restrictive core and excessive pipe work is less efficient than one with similar cooling but a less restriction.
The turbo is not working as hard to produce the same requested boost at the manifold. Prolonging the life of turbo.