(The better late than never edition)
I'm sure many of you have worked on web-apps (you know stuff like wikipedia, gmail, facebook, codechef, reddit, digg, duckduckgo or even your personal web-site which has some dymnamic/interactive content). I'm sure that out of these many, many of you have worked on sites that get traffic to the tune of a few hundred visitors per second. I'm sure you know how painstaking it is to run such a site and keep it up and running and responsive for all its users. Lots of things like the web-server, DB, file system, OS, etc... need to be tweaked and tuned and retro-fitted to suit the needs of the things around it.
The one place (in my experience) that can easily introduce bottlenecks is the DB (database). That's because it is something that is a real work-horse and is programmed by you (the application writer), who may not know what food it likes. It's very easy to write a query that runs 10x slower than what it really should be running and not notice this for a very long time.
Imagine the time you get slashdotted and need to support 5000 requests instead of the customary 50 requests/second. Don't you wish your DB doesn't just die due to the load?? Well, if you are one such person who is pedantic about the performance of your database, you are thinking right!
You will mostly learn how to optimize your DB through experimentation, but since others have been burned before, you might as well stay and take advantage of their experiences.