I used to be skeptical of people who lunged before they got on, but now I do it with my horse and I get it. It's just safer that way for a lot of rides. My horse, for instance, just turned 4, is a welsh/trakehner, and ridiculously athletic. Athletic to the point where he could bronc you off without putting his head down. He's also a bit of a moron. 8/10 times when on the lunge he does something stupid, like a sliding stop, a spaz/bronc, a crow hop, just something, so it's just better to lunge him for 10-20 minutes (whatever he needs) so he can get the sillies out and get his brain in gear. While I was away a month ago my trainer was riding my horse and didn't lunge him before she got on one day, and in the first 30 seconds of her ride was bronced off and broke 5 ribs had a blackout concussion. She's $25,000 knee high in medical bills, (thank god she's insured) and just getting back to driving her car a month later. So I'll stick to lunging.
Lunging is also great for just getting your horse to stretch out and get warmed up. I used to know a dressage trainer (Olympian who I thought was the scariest person in the world, "unconfident" would be the LAST word to describe this lady) who lunged all of her horses before anyone got on. She even taught the horses that if she ducked the horse was to buck to stretch out their backs. I don't know how everyone else lunges, but for me, lunging isn't about making the horse run in strung out circles until they sweat. When my horse goes out on the lunge, I expect him to quietly walk while engaging all of the right muscles until I think he's ready to trot, and when he's trotting, I play with adjusting his gait so his he's constantly thinking and constantly working, not just running on the forehand. I barely canter him on the line.
Those who don't have hot horses, that's fine. Just try to understand that for every seasoned ride there's just as many green and/or hot horses that need a different routine to be safe.
And then for those of you who think people who lunge don't know how to ride or aren't "confident" enough to knowingly take a dangerous risk, bless your heart your trainer is a rodeo cowboy.