Why indeed? The practice isn't standard among local dive boats in the U.S. where divers bring their own gear and usually even their own tanks. Everyone sets up his or her own stuff and the DM stays on the boat, maybe saving someone if they have the decency to try dying on the surface. On the other hand, the practice is very standard among tourist dive ops where many or most divers may be renting gear, because the divers show up at the boat and the gear is already set up.Meng_Tze:Let me turn the question around, why would one let others set up the rig?
So what about those of us who are used to setting up our own gear at home when we go on vacation, why would we let someone set it up? Sometimes, it's expedience. Here's an example: I've dove with Ed Robinson's on several trips to Maui. When you meet them at 6:30 a.m., the boats are sitting on a trailer in a parking lot. You hand your gear up to a DM on the boat and then check in and stand around drinking coffee until they put the boats in the water. They like to take off right away, and often it can be a bumpy ride. It's more efficient for them if the gear is already set up by the time the divers hop aboard. They've never had a problem figuring out the quirky upside-down topology of my reg, even with the one-from-each-side SSI configuration of the second stages, and even distributed the weights the way I like them.
When dive ops keep the gear for you overnight for multiple days of tourist diving, you'll show up the next day and the gear will already be set up for you. Again, it's efficiency, since you're ready to go as soon as you arrive. All it takes it a few quick checks to make sure everything is OK.
Other times, it's just a sincere offer by someone in a third-world country probably poorer then me who sincerely wants to earn a tip. In that case, I'll watch them set it up, and if there are any glitches, I can point them out and correct them along the way. This can lead to friendly exchanges of information and a more rewarding experience overall than the reception I might receive if I yelled "hands off my gear, dammit, no one touches it but me, especially none of you dirt poor third-worldians who are gonna screw it up just like you screwed up your entire country!" and followed it up with a slap to the face of the offending DM. (There are situations where the latter technique is warranted, but it's not usually a good way to start off)