Lets imagine that you have an Experiment that is targetted to a particular audience or multiple audiences - Custom Visitor Attributes allows you to apply those Audience criteria to your preview to enable you to see what those audience segments will see. I would imagine that these targeting criteria are only available depending on the type of license you have (silver, gold, Platinum).
Does that help you at all?
These articles on https://help.optimizely.com might provide more context on Audience Attributes:
Preview Mode: https://help.optimizely.com/hc/en-us/articles/2000
If you are on a plan that does not have geotargeting for example, you may choose to enable Viewing As coming from San Francisco, but because you are not able to create an Audience and target an experiment to only visitors coming from that location, when you apply that attribute in Preview mode, your variation won't change. Viewing As only applies those attributes to the Optimizely snippet and not to the overall website.
Solutions Architect | Optimizely, Inc.