"How can something made only of air hurt you? Easy. Air elementals are creatures capable of manipulating every particle in the air and conjuring bolts of lightning out of clear sky."
As with most level 2 and 3 summoning spells, the number of air elementals you can summon with this spell probably isn't enough to really affect the battle. Don't cast this unless you're short on melee support and you don't have any better summoning spells available.
Probably the best use of air elementals is to quickly engage dangerous ranged troops in melee so that they cannot fire, nor can they retaliate to your own ranged attacks. Because they are also Insubstantial, they are pretty resistant to other than spell damage, making them good barricades. They're also very mobile, which adds up pretty well to this strategy.