I'm not sure I got the question
But simple use case is this:
Let's say your user uses picasa as well and marks photos with tags like your, family, friends and others. These tags are written to photos using exif (AFAIK).
When using tag2find user want's to find these photos as well.
Main idea is this.
Let's say user went to Russia and took 100 photos, wrote 10 gps tracks shot 10 avi's, recorded 10 sound tracks.
User can set jpg tags using picasa like "tour,Russia" but actually user want's to set tags to all the media he/she got from the tour and should be able to find all the media content about tours using "tour" tag.