Airbags are great, but they are definitely not a replacement for a Weight-Distribution Hitch. A WDH has 2 primary purposes, as well as several additional benefits.
A WDH will raise up the back of the tow vehicle, and reduces some of the sag caused by the trailer’s tongue weight. And I think this is what the OP is referring to in his comparison to air bags. Both will raise up the rear end of the truck.
But the way a WDH raises up the rear of the truck is key – it uses torsion to shift a percentage of the tongue weight to the front axle of the tow vehicle. This is the key difference between a WDH and airbags. Without shifting some of the tongue weight onto the front axle, you‘re likely to have problems in a panic stop or other emergency maneuver. When you brake hard, the weight of the trailer shifts forward and pushes down on the hitch ball even harder than normal, which causes the truck to get light on the front wheels. When that happens, you can lose steering and a big part of your braking capacity. A WDH resists the trailer’s nose-dive, and keeps more of that tongue weight transferred forward.
By using Airbags instead of a WDH, in the same panic stop as above, the trailer weight will shift forward, pushing down on the hitch ball. The hitch in turn, will push down on the rear of the truck. Since the airbags are resisting that downward force (at the axle), they become a pivot point, and the front end can get light very quickly.
In addition to the 2 primary purposes described above (shifting weight to the front axle & reducing sag), a WDH also introduces more stability when encountering cross winds, especially if you use a sway control device with the WDH. Also, most modern trucks come with hitches that are rated lower when not using a WDH, and higher when using one. Some peope ignore those ratings, but that's a whole other conversation.
Hope that helps.
2006 Black Unlimited 6-speed, Rubicon Express 5.5" Long Arm Kit