We know that in the era before the Avatar, the Lion Turtles and/or humans bent not the elements, but the energy within themselves. We know energybending can give or take the ability to bend, allow the Avatar to leave their body, and create new spirit portals. We've seen the ancient Lion Turtles use it to give the ancestors of modern benders the ability to bend in the first place.
But when, how oftern, and why was energybending used in the era before the Avatar? What did people/Lion Turtles actually do with it?
Was it used for normal, casual, everyday purposes(like healing, maybe?), for battles, what for?
I think that energybending could have been used to transfer information between people. For example, the lion turtle that Aang met is never shown explaining HOW you can remove one's bending. All we see is the lion turtle using the technique on Aang, then Aang later successfully using it on Ozai. I believe that the lion turtle, by using the ability on Aang, also taught him how to use it himself.
Now if that is true, what are some scenarios where transferring information through energybending is practical?
During wartime, it could be used to extract information from prisoners if my hypothesis is correct. By performing the technique, you could learn everything that the prison knew. The locations of strongholds, the supply lines/routes of the enemy, any code words and their meanings. The list goes on.
It's never clear in the Canon if any other languages are spoken, but I'd wager that you could teach new languages through this process. Not only that but you'd be able to instantly give cultural context as well, allowing the recipient to have a better understanding of the language and it's people. I believe that this particular application would require the practitioner to themselves be fluent in the language and culture they are trying to pass on.
It could be used in healing to give a practitioner a thorough reading of an injury to in turn provide a more detailed diagnosis.
I believe that energybending has both internal and external applications however for practical used I'd say these would be my top scenarios.
Well first of all, information transfer through energybending has so far been shown as a one-way thing. You have to already know the knowledge you're attempting to implant in someone or yourself. In addition this energybending technique has only ever been shown by one lion turtle which (it can be reasonably assumed) might have specialized in only energybending. As for what energybending techniques can be done, I honestly can't really say because of how little of the series deals with this.