I used to only train in one particular rep range 10 to 12. This range is commonly held to be the optimal range for muscular hypertrophy and development. However, studies show that by using a variety of rep ranges with corresponding adequate resistance increases gains by triggering various mechanism in muscular development. That's fitness geek talk for variety is the spice of life. Since spicing up my life I've broken through a plateau and killed boredom. I now love my heavy, low-set days.
There's a couple different ways to incorporate this form of undulating periodization (more geek talk).
Cycle through heavy, medium and light programs, each lasting about 4 to 6 weeks.
I personally prefer cycling through each rep range per workout. Eg: Monday low reps and heavy weights; Wednesday medium reps with medium resistance; Friday high reps with lighter weights.
Which ever rep range you choose, make sure the weight is heavy/light enough so you are maxing out by or before the top end of the range.