Helps develop the proper muscles for explosive power. When you need to lunge to get that tight drop shot, or jump to the back of the court to get that offensive clear. I would typically set my weights such that I can do 3-5 sets (depending on my time) of 10-15 repetitions.
keep in mind the proper technique when doing squats and lunges, the knees should not go past the toes this will help to isolate the muscles and also not put strain on your knees
Oooh, a feisty one...not only did you not acknowledge druss's answer to your question, but you're ready to snap at someone else providing some good advice.