NVR's that support ANR will retrieve any missing footage from the cameras memory after an outage; this allows you to update firmware or take hardware down for maintenance late at night w/out worrying about missing something.
Murphy's a bitch, I bought a new HDD a while back and went to go install it at 1am.. while I have the NVR disassembled and in my lap I get a notification my mailbox was opened and looked at my display to see someone trying to steal mail.. I chase em off and come back and realize nothing got recorded, then I ordered a bunch of memory cards and started putting em in every camera.
With 265 I can get ~4 days constant on a 128G card, not bad for primary cameras as a backup.. but I typically put 32G cards in and just the last day is really all I need, if someone jacked the NVR I'd still be recording and have a day to stop recording and retrieve backups, if the jacked the NVR and took the entire network offline then I'd have the last day up to the outage which should include a large portion of the events.
Also after a natural disaster, even if the camera is found smashed blocks away the flash memory typically yields results right up to the event that are at least news worthy if not good for insurance purposes.. HDD's are far less likely to survive a natural disaster, they wont tolerate any abuse.
sometimes shit happens, mebe the network dropped a few critical frames or the NVR hit overload or auto-rebooted and having a local copy avoids heartbreak when its time for your video surveillance to finally shine..
lastly, If your using a camera on WiFi local flash is really the best recording option available.. uses up no bandwidth and dont require a stable network connection.