It's far far harder for a faction to defend itself when their enemies have massive air support and they do not.

Isis lose with or without boots on the ground if their current enemies in the region are simply given heavy air support and supplies.

Putting boots on the ground, Benefits:

More local control.

Better trained soldiers.

Cons:

Expense cash.

Expense lives.

Needing to run an occupation.

Difficulty of nation building due to resentment against your new state from local peoples about foreign occupiers.