It all depends on your goals, and the reason you are lifting weights. Low reps (up to 5 build strength), higher reps build endurance and hypertrophy (muscle growth). As your sets of 15 reps increase, your 1 rep max for that lift will also increase, but you are not specifically training for a 1RM.
That's correct for working out body parts like your arms, shoulders, and chest, but from my understanding your legs require a higher 'minimum set' for growth stimulation. The thought behind it is that your legs get more of a workout on your average day just from carrying your body weight around that in order to see your best gains you have to do more reps than you would with other muscle groups. So for strength gains you'd be doing something like 8-12 reps, for general fitness you'd do do 12-18, and for endurance you'd be doing 18+.
If it's wrong then (honestly) please correct me, but I've been doing this for around 5 years now for my leg workouts and it has worked really well for me.