Modern applications are, at least to some extent, data-rich. What this means is that often times applications will have features like the Twitter Feed, aggregated statistics, friends/followers and many other features that rely on complex inter-related data.
Its this data that provides the vast majority of the application value. Twitter would be quite useless if the only thing you could do is post and could only see a handful of others' tweets.
The biggest pitfall most developers can fall to is re-using the production database for development
Due to the complexity of this data, during the early development process of an application, its tempting to use the same database for production as for development.