In short: because the value of content without meta-data is very small. In greater detail: because contents supplied with meta-data are easier to find later.
Because contents found this way are reusable.
Because this way a collection and its documents become visible and usable for everyone. Because this way a collection is really a collection, not a useless pile of data. Because meta-data equipped contents are easier to interpret automatically, even on a semantic level. Because contents with uniform meta-data can be merged with other meta-data equipped contents. Because meta-data equipped contents are very easy to reuse. Because meta-data equipped contents are entered into the NDDA search system, becoming, accessible, visible and available for structured querying.
In shortest: because making meta is good.
