Planning is used for both navigation and task planning. For navigation, path planning at a high abstraction level would probably ignore small objects and many details of the environment.
In addition to the replanning that can be done in the case of failures, the architecture's tasks are interruptible. This allow for a hierarchy of importance levels, which is important when you are approached by a tiger while cooking in the kitchen. There are can be cleanup problems involved with interruptibility. ATLANTIS supports cleanup, and also allows cleanup to be ignored or interrupted if an event or observation is sufficiently important.
Go to a discussion of this capability for multiple architectures.