When Apple released iOS 10, it finally allowed users to delete stock apps, instead of banishing them to a junk folder on your device. Now it appears Activity is also joining in on the fun. The Activity app only appears on your iPhone after you pair Apple Watch to your device. If you unpair your watch, the Activity app sticks around and currently cannot be deleted.
One area Apple needs to expand is to move Apple Watch backups to iCloud. Currently, users need to sync and backup an Apple Watch manually before they unpair the device.