git annex export normally exports all files in the specified tree, which is generally what the user wants. But, in some situations, the user may want to export a subset of files, in a way that can be well expressed by a preferred content expression.

For example, they may want to export .mp3 files but not the .wav files used to produce those.

Or, export podcasts, but not ones in a "old" directory that have already been listened to.

It seems doable to make git annex export honor whatever preferred content settings have been configured for the remote. (And git annex sync --content too.)