I googled your question, here are some of the answers I found...
with most modern AV programs and Norton in particular you must be connected to the internet for the scan to run since it depends on simultaneous checking with the most recent records that are online.
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Certainly with the new product Norton Security and Norton Security with Backup, since they require an internet connection to access the latest cloud definitions.
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I found if I try to run a scan without a connection I am only permitted to run a quick scan, a message that says connection to internet required to run a full system scan pops up. Instant check with latest definition's and a check to see if a particular file has been made from adding a variation to the signature of a know virus is the benefit of that "cloud" scanning. Panda AV was one of the 1st to invent this method but now all the AV makers do it.
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And here's one for wehateporn...
I think a National Security Agency spyware virus would maybe benefit from an offline scan but companies like Norton are required to enable some NSA spyware as part of national security. A foreign company like Kaspersky might get around that but if cyber warfare breaks out your computer would be 1st to be accessible to the home country of Kaspersky.
All of these are on the Norton Community forums.
If those answers aren't quite to your liking there are only about 90,000 other results that might help you...
https://www.google.ca/search?q=Why+d...QAg&gws_rd=ssl