This field is missing when try to and a predicate, while we need it to search on the blob's key.
Main use case: Search for duplicates (search all documents, not versions, not proxy, whose blob is the same). For example, this search works fine in WebUI > Admin > NXQL:
SELECT * FROM Document WHERE file:content/data = '123456abc...' AND eco:isVersion = 0 AND ecm:isProxy = 0
It is also misnamed (should be key instead of data, but this is likely a ticket for NXP)
There are several workarounds to search on blob's key. One is to denormalize: Add a field to a schema (like my schema:blobDigest), and update it when the document is created with a blob and when the blob is modified.
Now, this workaround is required wfor every field holding a blob, while exposing data will make it way easier to search also in custom fields.
Also, note that (as Florent said) "you should add an index for that field in production if you have large volumes"