Mega WordPress Applications WPMU or otherwise

I’m a small-timer, so I was amazed at this question: Ideal WP multisite server setup for up to 1000 sites?. I know that WordPress is a highly flexible platform, but what types of entities/businesses would need or use this type of setup? In situations like this is there much redundant content? How do you begin designing something like this? Are there lessons in this for the small-timers?