When to use context.session

[#1]

From the javascript object model doc (http://docs.apigee.com/api-services/reference/javascript-object-model), there are two ways to pass objects to different policies: via custom variable or context.session. All the other documents I've read mostly just use custom variables. What is the use case for using context.session? Any advantage or tradeoffs with context.session?

Thanks,

-Wei