For me it was back in vanilla actually, when I was running quite a lot of BG's right after they were added to the game and I started realizing the huge difference that having a good healer made on the overall experience and obviously on the match results.
Sometimes we couldn’t even find a healer that wanted to join for some of the games, I really didn’t get why everyone wanted to be dps, but then again… I was dps as well!
So I decided to go ahead and create my first healer character with the intention of going PvP only at max level, and it was such a blast! There were so few healers in BG's back then that other players kept thanking you for healing them, it really felt like such a powerful role, and a lot of times it made a clear difference between winning or losing. (especially when the other team had no healers! B) ahah!)
I Still remember to this day how epic some battles were, and how cool these Field Marshal's Satin Mantle
looked on my dwarf priest when 2.0.1 went live.