You need airstones so that The fish can breathe, even though Bettas don't nessecarily need them, they do when the temp of the tank is in the 80's because like you said when it's higher there is less oxygen for them to breathe. It also helps with circulating the water around to give an even temp in the water, so it's not too cold in one area and too warm in another . If you intend to have more then one fish in with the
betta, you will need one anyway because not all fish can breathe oxygen like a betta can. Which I'm sure you knew that already.

Natalie
P.S. I hope that explanation helped.