Nursing pillows are good support for one baby or multiples. They allow you to get the baby up, closer to the breast. Many nursing moms often are sporting poor posture because they are slumped over in order to get the breast closer to the baby. To get a good latch and ensure the baby will learn to nurse effectively and efficiently as well as helping mom maintain good support, you need to bring baby to the breast, not the other way around.
Even if you start holding baby nice and high, often slipping occurs which pulls on the nipple and affects the latch baby has on the breast. Both of these things can cause damage to the nipple tissue and make the nursing relationship painful.
If you have been able to effectively nurse your baby without a pillow, that's great. I just would encourage you to make sure your posture is good. Nursing can cause muscle cramps etc... in the upper back. Baby gets heavy while you hold him/her and even when you don't think you are slipping you often are slipping. Having never used a nursing pillow, I'm guessing you have not experienced the increased comfort that comes from using one. It's a tool. Just like anything else. Is it NECESSARY? In most cases, no. But it does make life easier.
And, as others have said, it is an huge help for moms who have had a c/s.