Originally Posted by polkweed

The foundation stabilizes the fabric to prevent stretching. Less important if you're using only quilting cottons, but a big help if you're using a variety of fabrics, and a must if you're doing a selvage quilt. Also you can use it as kind of a paper-piecing template if you want a really precise pattern.

And because I don't think anyone has mentioned it yet, you can also do the quilt-as-you-go method and use your batting as the foundation. It saves you the extra weight/thickness of a foundation fabric, and you can swap the batting for felt to go even lighter.