I had never heard of these needles, so I looked them up.
http://janome.com/en/blog/introducin...e-tip-needles/ They are designed to prevent skipped stitches when sewing on heavier fabrics, including the layers in a quilt sandwich. You wouldn't use them for ordinary piecing. It's pretty typical to need a different needle for piecing and quilting, or to need different needles for different threads. Often a needle with a stronger shaft (which bigger needles typically have) will prevent skipped stitches, because it doesn't flex as much. This wouldn't necessarily have to be the purple tip needle.