While it can be expensive to send just air units against ground units, sometimes it is warranted.
In a recent game of 1942 2nd edition, Germany was really giving Russia a hard time and Russia was having trouble having enough troops and tanks to retake their own territories. The US and UK had built up sizable air forces so they took turns attacking the German infantry and tanks so on Russia’s turn, they could simply move in and take the territories thus improving their income.
While the US/UK did lose some expensive air units, it worked out very well as Russia was able to roll up a lot of territory and Germany just couldn’t produce enough to reverse the tide. Russian tanks would blitz 2 territories while Germany could only take the 2nd territory back in combat. Then here would come those dang Brit or American planes again.