Twin needles come in a variety of sizes. They can really make those built-in stitches on your machine look more interesting (especially with two different thread colors). Don't forget the double needle that has a regular needle and a wing needle for heirloom stitching (it looks really nice).
If you are wanting to make a stained glass quilt you can use the twin needle to stitch down the bias tape. It works best on straight areas and really gentle curves.
This is one of those things that you really should play with because you will be surprised at the look.