>I've always wondered why this is desirable

Others have noted other reasons, but I think the biggest reason is that 1st/business class seats are more comfortable than the crappy crowded boarding area. Economy seats, it's the other way around. And as an added bonus, on something like a 787 where the boarding doors are behind business class, boarding first means you're lounging around in the front of the plane being served drinks and addressed by name while behind you the rabble are are being herded like cattle into their tiny seats at the back.

For 3x the price, 1st/business class want luxury, and sitting elbow to elbow with the riffraff in beat up terminal seating isn't that.