If you have multiple on-line identities or personas and you would like to ensure that there is no cross-contamination of personal information between them, then this is the extension you need. Likewise, if you want to ensure that Microsoft knows nothing about your preference for Mozilla products and services. Same goes for any other organisation, such as Google or Amazon, that likes to collect and analyse your on-line activities. You can define individual containers to keep your activities with each organization isolated and encapsulated, completely separating them from each other. What's not to like?