In the film Avatar, we see the human troops using their air superiority to good effect when they attack Home Tree. They're then challenged (in the air) by various raptors and flying creatures.
At this point, they land mech-warriors and heavily armed ground troops and fight a battle on the ground as well as in the air.
Admittedly it made for an exciting sequence, but why use men and material on the ground at all? What was their objective?
On the face of it, the objective of the ground assault was to:
Taken together, these would dissuade the Na'vi from any future attempts to zerg-rush the base.
Quoting from the original script:
QUARITCH: These orbital images show the hostiles' numbers have gone from a couple of hundred to over two thousand in one day, and more are pouring in. By next week it could be twenty thousand. Then they'll be overrunning our perimeter here. We can't wait. Our only security lies in pre-emptive attack. We will fight terror with terror.
That being said, the decision to go for an all-out assault (air and ground) seems to have been taken by Quaritch personally. The decision of the Na'vi to attack the base gives him and his troops the pretext he needs to overkill the Na'vi and while there's a good tactical reason for the assault, it seems that the desire for revenge (at the Na'vi's persistent attacks on the miners) is pretty damn strong too.
SELFRIDGE: Listen to me! I am not authorizing you to turn the mine-workers local into a freakin' militia!
QUARITCH: I declared threat condition red. That puts all on-world assets under my command.
The OP's question seems to indicate that the ground troops are dropped after the air forces are already under attack. This is not so. The land forces are dropped prior to any confrontation, and in fact, they enter combat before any counter air assault by the Na'vi occurs.
While "Plan A" was to drop the explosives to level the sacred tree by the air assault, there were unknown factors. They were going into a region that scanners and instrumentation did not function well, while flying a large shuttle through an aerial obstacle course to get to their prime target (recall the comment about loosing some "paint" by the shuttle pilots as they enter into the floating mountains). The lack of instrumentation for targeting may be why they chose to "drop" explosives rather than just missile it to death like Home Tree. This is another oddity about the strategy; why not just fly all the aircraft close and rain missiles down like the Home Tree attack? (I do not have a good answer.) Nevertheless, that was the primary strategy.
However, given the speculative aspect of being able to navigate the larger shuttle in through the mountains and succeed with the drop of explosives, likely the ground forces were a "Plan B." If the shuttle could not make the drop, then Plan B was probably to close in on the tree and take it out at ground level; recall, it was magnitudes of size smaller than the Home Tree.
