Searching, as Joe notes below, is one thing.
The real "killer app" is find.
It's the success of the "ultimate Linux app," Google, that has everyone and his rich uncle in Seattle going after this application.
Joe's question remains. Why?
The answer, I think, lies in Moore's Law. Moore's Law of Storage (as I call it) shows hard drive capacities growing bigger-and-bigger, faster-and-faster. Many people never unload their drives, their caches, or their old stuff -- at $1/gigabyte and declining why bother? (Most don't even back it up, except perhaps to copy the drive.) All this is true, in spades, for Internet servers.
So better search algorithms, and a more accurate presentations of results, are both devoutly to be wished. Google concentrated on search to build an Internet business, but as hard drive sizes continue ratcheting out of control it seems the best route to building any business.
Find my stuff and I'll give you anything, because I have too much stuff.