If you are new to loom knitting you will want to be sure to check out
How to Make Knifty Knitter Stitches
It explains all the stitches that can be made from a knifty knitter loom, including the purl stitch. It also gives step by step instructions for how to make the stitches.
You can make anything with a simple purl stitch, however, some other stitches are often incorporated into large projects, like a blanket, because they help them go faster and add a little variation.