People who are bullied will often internalize that they are worthless (and that's why they are bullied). That can be one of the hardest parts of being bullied.
This happens especially if people who witness the bullying stay silent. Than the bullied will feel even more alone (and will feel awful about how bad they feel).
But if witnesses stand up and say- "hey, bully: you're being an asshole", then that is a service to the bullied. The bullied will at least not feel so alone.
Hope that makes sense.