Personally, I lunge my horse when needed, mainly if I'm worried about how much energy she'll have, like today. My job has been crazy, have only ridden her once in the past week plus - lunged her for ten minutes to get the buck out. She was still very fresh when I climbed on, but no more rodeo antics.
My trainer (H/J A/AA barn) will also use it for conditioning a horse. She will use a training system (can't remember what it is called) that helps with framing a horse up properly and encouraging proper use of their body. She will use this a lot with greener horses and those coming back from an injury.
She will also lunge when the rider needs time on the lunge line for their own education. Then of course when the horse needs some energy out before a ride.