Turns are often called "exploration turns", and are used on a different time scale, often to track movement around the dungeon (movement in dungeon exploration is often given "per turn"), it takes a Turn to search a wall, light sources last a given number of Turns. A common assumption is also that each fight takes one Turn, even short ones (a bit abstract, but basically it includes a bit of rest and recovery after the fight). A lot of non combat spells have durations given in Turns, as well.
Basically, they make it easier to track time in the dungeon, which was very important in the "old days" (and, to me, still is). 
I use a special time tracker with tick boxes for each turn, where I can also note when spell durations end, when to roll for random encounters, when light sources run out, etc...