Just from my experience Tobra(IV) is used during hospitalization when I've gotten sick and need a more powerful regime. TOBI(neb) is part of the regular routine to help prevent or slow down illness. My doctor's have given me Tobra even though I am using inhaled TOBI before and I asked the same question about why and what was gained from doing both.
The explanation I got was that TOBI helps by contact with the outer layers of mucus that air can come it contact with, which of course can be limited with us. The IV Tobra can reach the inner layers because it goes through the blood and can get inside the lung tissue and thus get to mucus that is not exposed to air. My explanation of the anatomy may be a bit off here but hopefully you get the idea. Attack it from both sides.
The explanation I got was that TOBI helps by contact with the outer layers of mucus that air can come it contact with, which of course can be limited with us. The IV Tobra can reach the inner layers because it goes through the blood and can get inside the lung tissue and thus get to mucus that is not exposed to air. My explanation of the anatomy may be a bit off here but hopefully you get the idea. Attack it from both sides.