Generally the advantages of landing are:

1 - Can drop off ground troops or tanks that can then add their shots to the attack.

2 - If the aircraft lands, it is eligible to claim Crossfire, reducing the target's armour save, which it can't do if it stays in the air.

3 - It can also do some stuff relating to victory conditions / objectives.

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In the final assault the thunderhawk was able to add two supporting fire shots as well

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And still be able to contest an objective to stop the Orks gaining the Take and Hold goal if the engagement failed.