What do people mean when they say "automation is a tool"?

In Emily O'Connor's first 404 talk, she discusses her understanding of the meaning of “automated testing is a tool”. 

When performing exploratory testing, testers learn a lot about the system under test, but by the time automation is written - testers are only putting into code fixed assertions, meaning that automation does not provide new knowledge and is used as a tool to save teams time searching for regression bugs.