Yes, you need a sleeping pad for your backpacking quilt. A sleeping pad provides insulation from the cold ground, which is essential for staying warm at night. Quilts do not have insulation on the bottom, so they rely on your sleeping pad to keep you warm.
If you do not use a sleeping pad with your quilt, you will likely get cold at night. The cold ground will conduct heat away from your body, and your quilt will not be able to keep you warm. This can lead to a sleepless night and a miserable hike the next day.
There are many different types of sleeping pads available, so you can choose one that is right for your needs and budget. You should also consider the weight and pack size of the sleeping pad when making your decision. Sleeping pads can be bulky and heavy, so you want to choose one that is lightweight and easy to pack.