It is always hard to describe why flickr is cool to people. It's not just a place where people threw up a bunch of pictures. Describing geotagging, searches by interestingness, and an open API, doesn't work to explain because they are better seen than heard.
But here is an application that is a good example of what is possible.
It shows recently posted sunset photos from wherever in the world the sun is setting right now.
To do this it,
Knows where the sun is setting right now
accesses the Flickr API
looks only at photos that are geotagged (the owner said "this was taken here")
of those looks for ones that also have the tag "sunset"
finds one geotagged in the band of the earth where the sun is setting right now
and pops it on your screen.
Simply because the sun is always setting.