@richardlee the easiest way to describe it is do you just care that B is better than A or do you care precisely how much better B is than A as well as the other experience variations? Auto-Allocate will find the winner and start shifting traffic over faster.
This white paper explains in depth: http://wwwimages.adobe.com/content/dam/acom/en/marketing-cloud/target/pdf/54658.en.target.whitepaper...