The monitor-only is indeed for assessment purposes only; however, you will be able to test out the action taken by the sfr module without interrupting any network traffic.
In monitor-only mode the module receives a copy of the actual packet and takes action on the dummy packet, under the FMC Analysis > Connection > Events you will be able to see the packet and details along with the action.
For example, lets say we create a rule to block facebook.com using the Access control rules and the policy is applied to the module. Now, if any user is accessing facebook.com that event will be shown as a "Block" on the FMC and you will be able to get details of the user without interrupting his access. In the similar manner, you will see "would have been blocked" events for any traffic triggering an Intrusion rule. Once you are satisfied with the rules and the working of the module you can switch to inline mode and start blocking/handling actual network traffic.
The sfr module handles traffic which is going through the ASA based on the service-policy. It will not be logging any access to the ASA itself; that has to be done on the ASA.
