Parley served a minor gameplay function in version 1.0 in that it was designed to be another way for Disciples of the Hand and Land to undertake story quests (read, boss battle fights), without actually having to fight (hence why it's called 'Parley' in the first place - it's technically analogous to having a discussion with another person, and in the context of the game, it was an attempt by the player to convince the enemy to give up without fighting). Naturally though, this didn't work well and thus was abandoned as an actual gameplay mechanic in ARR (hence why you can't start ARR as a Disciple of the Hand or Land anymore unlike in 1.0).

So yeah, now it's pretty much just a time waster, a little diversion for if you're bored and have nothing better to do.