It's interesting how much of this thread focuses on just one function of the DM, guiding tourist dives on tropical boats. I DM'd for a year and a half (very part time) and never did this. Around here, DMs are critical for safety in classes. Occasionally they'll guide divers new to the area.
The last class I taught had very low viz from 0 to 20 feet, low viz to 30 feet, and 25 foot viz deeper. Three students, me as instructor, another instructor acting as DM, and a DM candidate. The DM was critical to keeping track of each student as we moved through the murk to deeper water. My DMC was awesome as well, herding sheep from behind.
Similarly, if one student bolts for the surface, the DM either intervenes or brings the rest of the group up with him. Or if one student can't continue, they can be taken to shore by the DM.