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I don't believe it's a matter of online vs. offline, it's simply the means of creating a seamless, well-performing application between server and client with a local data store. Cookies don't cut it!

anyone assuming the network never goes down shouldn't be creating networked applications.

There are large areas even in the so-called "developed" world where internet access (and especially permanent connections) are scarce of non-existent.
Over half the people in continental Europe and the continental US that have internet access still use dialup (and in Europe that means paying by the second, so they'll go offline the moment they can to save skyhigh phonebills).
Many of those people don't have a permanent connection either because it's more expensive for them or because it's not available at all.
People living on islands and in remote regions may be restricted (as is a guy I know in Alaska) to a slow satphone hookup costing enough that he can't afford to be online for more than a few minutes per day, just long enough to download his email and upload his responses.
Others live in areas remote enough that there are phonelines but no broadband services so they're stuck using 56K modems on overstretches (and thus unreliable) networks.
We do business with areas where broadband is available but so expensive even large companies don't use it or if they do limit it to a very few machines that require it for receiving streaming business data.

Not everwhere is "megacity USA" with fibre optics into every room.