There are several reasons why a diver (or divers) might rerquire a Private DM. I'll start by saying that we have many divers who don't actually require to have one for a particular reason, and just do so because they like diving in their own group of 1 or 2 divers, usually with their favorite DM (whom they've been diving with for years).
Diving with a Private DM on a 2 tank trip, is required (by Scuba Du) when a person holds only a Scuba Diver Certification (in PADI that means they have not finished the full Open Water and can only go to a max depth of 40 feet), as our 2 tank trips do an 80 foot dive for the 1st dive, and the second can be 50-60.
When we have Junior O/W divers we also require a Private DM for the same reason, depth limits, 10 & 11 yr olds cannot go past 40 feet, and 12-15yr olds are recommended to stay above 60 feet. We do have the occasional 14 or 15 yr old (6'2, 250lbs) built like adults, with 100+ dives, and in those cases they are just required for 1 day so our DM can evaluate them, and if they're good divers and the parents so request, they can dive in the general group w/o a Private DM by signing an additional Jr.Diver Release. We take it on a case by case basis.
Other likely candidates for Private DMs are some very new and inexperienced divers, specially when they haven't dove in the ocean (certified in the lake 47ºF water, 7mms and 4 foot viz). Many of us forget what it was like to be a spanking-brand-new diver, all stressed out and nervous, trying to go over every single little thing the Instructor made us memorize, looking around at all these other divers, obviously far more experienced (giving us that "Oh no newbies" look), worrying about keeping up with the group, or holding others back, and how did this regulator thing go??? and oh my god look at that huge ocean!!!! :shocked2: Yes, we forget.... For many in this situation, having a Private DM is a blessing (well worth the extra charge), they can forget about the other divers completly, they have their own personal DM paying all his attention to them, hand-holding when necessary (and it's OK, it often is necessary, while a new diver builds confidence in their own skills), and diving at their own pace and leisure, you can't beat that for 50 bucks.
So there can be several situations where a Private DM is just the right way to dive...
Happy Bubbles!!!!!