<p>I see them all the time and always ignore them. Can someone explain to me why they have become so prevalent?  If I'm using a site that allows me to explore it via tags (e.g., this one, del.icio.us, etc.) that's what I will do.  Why would I need a "cloud" of tags upon which to click?  I can just type that tag(s) into a search box.  What am I missing?</p>

It's a easy mechanism to determine which tags are most popular or how dense that tag is populated ( amount of tags).
It's just a intuative interface, I'm fairly certain that's one of the bigger reason's why they are so popular, that and they are very Web 2.0 also.