Data often changes. When it comes to integration, the data can change unpredictably. Of course, large vendors notify the community about the breaking changes in advance or give us some period for migration. However, if you implement your own component of a (distributed) system then you likely face a situation when, for example, Tom changed something, while you were working on your cool feature of a Java-application and now you have to fix some unexpected failures on your side. Or a customer asked you to include some small (or big) changes in the initial specifications and next day Tom found out that his part did not seem to be working.