Honestly they all have their purpose in society when you think about it.

Earth (tied with water for #1 most useful imo)

• Basically free structure building. Like any structure almost.
• End homelessness by having homes everywhere.
• Landscaping and farming implications.
• building roads and bridges
• Metal benders to boot. Even better for structure building and any other metal working. Easy Magnet production AND FUCKING TRAINS removed.
• Added bonus if earth benders would be able to separate soil pollution
• Also hurling boulders gotta be pretty fun

Water

• Free housing as well if you like the cold (I do)
• Water scarcity is a big deal and waters can probably purify water. In fact it’s probably more likely water benders could help with environmental polution more than any other bender. Even taking it from soil through ground water.
• Healing is pretty baller. No surgeon would want to operate without some water healer backup and most all water benders can learn it.
• Got that magic “calm down angry spirits from killing everyone” ability.
• Never have to worry about drowning
• Bloodbending, while super rare and hard to do is super powerful and I bet could have amazing healing pote trial if used right.

Fire

• Can cook whenever
• Can actually be better firefighters than water benders cause they can just quell the fires straight up without need for water though probably takes lots of concentration.
• Fire benders are shown to be able to diagnose illnesses better than water benders and have their own healing abilities. Just not as versatile and easy to learn as water healing.
• Lightning benders would basically drive the world as it’s essentially free, clean energy.

Air

• uhhhhhh idk you get a cool Bison pet?
• In all seriousness though air benders probably the most in touch with nature and philosophy. So from a spiritual sense that has it’s purpose they were based off Budist and Taoist monks. I’ve followed the Tao for a few years and it’s a great philosophy. Even if you don’t live by it just understanding it is great.
• Can fly so that’s always cool
• I bet a bunch of air benders could work together to change weather patterns to bring rain to drought stricken areas, move rain out of flooding areas, stop tonadoes and other natural (storm based) disasters.
• If air benders could pull polution out of the air that would be amazing. Maybe not like CO2 and stuff but like, smog and particle pollutants might be possible.
• Also I’m sure they could be of use in certain industrial fields. Diverting air through pipes and machinery and stuff. Idk.