Your pool may be the location for the system dataset, where various system related files are stored. Those include samba runtime information and system logs. Especially logfiles are written to constantly. If you have the newest FreeNas version (9.2.1.6), you can disable using the ZFS pool as a target for system logs. You can find it under "System" → "Settings" → "System Dataset": Uncheck Syslog.
Note however, that using the pool for system logging is not necessarily a bad thing. Keep also in mind that logging to the usb stick (if thats where your FreeNas installation is located), weares out that stick faster.