In low level dungeons marking helps DPS know what target to attack first. Things go a bit faster probably. Unlike tanks and healers, DPS don't see all mobs on the enmity list until they have attacked them. Some players may be high level doing roulettes but sometimes there is a newer one. So it helps DPS to learn to concentrate on the same mob, or know who has to die first.

When the difficulty of the place is not low for the group, it helps. Except in huge pulls with lots of mobs (with AoE attacks mostly) mobs die quicker if there are marks.

Usually a 1 on the first mob you can change when it dies is enough, but marking 3 is also nice. If everybody knows very well the place it may not be needed at all.