Being able to search collective content by typing in a tag word can help narrow the search for the right image or video you may be looking for.
The creation of memes and mash-ups bring up the question of copy right and how much of someone's work can be used before it contravenes copyright rules. Then there's the question of stifling creativity. Creative Commons goes some way to helping with that score. You can use work to certain degrees tagged as a Creative Common, and if you are creative you can get a creative common tag put on your work and share it with others.
Kennedy, L., Naaman, M., Ahern, S., Nair, R., & Rattenbury, T. (2007). How Flickr helps us make sense of the world: context and content in community-contributed media collections. Proceedings of the 15th international conference on Multimedia. Augsburg, Germany.