[HDi fun]

It's not so much as controlling boost in steady state (ie. fixed speed driving) that VNTs or VGTs are used. At a steady 110mph the car will provide constant turbo pressure anway, for example.

The variable geometry is there to alleviate the age old problem of lag so that the turbocharger does less to impede exhaust gas flow at very low revs yet spools up more rapidly when you command more torque with your right foot.

I sometimes struggle with the 'boost' word. Seems to mis-represent the reason for supercharging cars.