Aside from the fairly obvious easy to organization another important part of it is when you're using sites like flickr.
Plain and simple: It will help get your image seen.
Last year I had someone contact me via flickr asking if a particular shot of mine was available in a larger size and am I willing to sell usage rights for the image. After doing some research on the person/company making this offer it turned out to be a group working out of British Columbia. The company was actually pretty prominent in the advertising world.
To make a long story short I ended up selling the usage rights to the image to them, twice to be exact. They had wanted the image to be a part of promotional material being put together for the Government of Alberta promoting the province of Alberta. The second time they had contacted me was to extend the rights to posters and billboards, something that was not mentioned in the original usage rights.
Companies watch sites like flickr and do contact users. Tagging your photos with appropriate keywords plays a key role in having your photos seen on such sites.Spent the extra few seconds to tag your photos properly. It might make you some money or bring you attention in the long run.