Several:

• when using FORK you can customize the JGroups channel by adding additional protocols on top of the stack defined in the main protocol branch, so we can add for example one which is needed for efficient auto-incremental keys which is not included in the default stack provided by the application server
• we would fully own a CacheManager: can inject custom services, override components, etc..
• we can control the configuration of our CacheManager
• you don't risk data conflicts with other applications using the AS's CacheManager
• you can use a different version of Infinispan than the one bundled with the container, but still avoid starting an ad-hoc channel (assuming they use a compatible JGroups version) i.e. could be useful for minor upgrades