Use case: I sync local data with web API. I do not use all the data I receive, only a few bits, but if I modify them, I have to send a complete object back to the server with all the other data. The simplest way to do this is to store the original JSON.

The CardDAV and CalDAV are not JSON, but their specification also requires you to preserve the whole vCard if you ever want to send your changes back to the server. CardDAV data may be accessed by multiple apps and they are allowed to add their private properties; any app that deals with vCards must preserve all properties, including those it doesn't understand or use.

One common reason is to provide a flexible table for things that may not have an identical schema. For instance, an event log might have details about each event that differ based on the event type.