We had lab coats for nursing school clinicals, it was optional to wear but if you chose to wear a lab coat it had to be the school issued one. I always wore a lab coat, during class time and clinicals.
Now that I am out of school and working, I always have a lab coat with me, for several reasons (some already listed by other posters). I usually wear a plain white lab coat, no name or alphabet soup after my name (then again LPN is only 3 little letters hehee)
1. I need more pockets than what my scrub top has
2. I tend to get cold easily but don't like wearing a long-sleeved top under my scrub top because its not easy to take it off if/when I get warm again. Its much easier to put the lab coat on or to take it off when needed. Add in that the rooms in my LTC all have seperate temp controls so one room might be like the Artic while another feels like Jamaica in August, its just easier to take the coat off if I'm in "Jamaica" or put it back on when I enter "the Tundra"
3. That lab coat has saved my bare skin from getting hit with all kinds of bodily fluids. For example, not long ago I was doing a blood glucose check on a resident, she tends to be "greedy" with blood so i always have to milk the finger to get the amount needed. This one particular day, I pricked her finger w/the lancet and proceeded to gently milk the finger, all of a sudden, blood shot out of her finger like an arterial spray, the blood went up my entire arm and onto my chest. Had I just had on my scrub top w/no lab coat, her blood would have been all over my bare skin...ewww, rather not have that happen.