The septum clamp is a tool designed to help piercers achieve a perfect piercing by providing a clear guide. The septum is notorious for being one of the harder piercings to get perfectly straight.
Many talented piercers do not choose to use them; not using one is not indicative of a poor piercer, nor is using one indicative of a good piercer. It's simply a good tool; some may like it, others may not. In addition, it's not a guarantee -- you can still do a crooked piercing using one.
The tool is a standard clamp (usually a modified towel clamp) with a tube welded to the ends. When the clamp is closed, a needle can be cleanly passed through the two halves, making it very easy for the piercer to do a accurate, tissue-perpendicular piercing.