This page provides links to two very short videos that provide a good introduction to SPC, with a focus on why it is a useful method.
These videos were provided freely on the internet, and are used as provided.
Short introduction to SPC (Statistical Process Control) by Gemba Academy. (~8 minutes)
If this link doesn't work in one browser, try another.
Really, really short introduction to SPC by PQ Systems (a provider of SPC software). (~3.5 minutes)