Where one piece of code depends on another piece of code -- whether by inheriting from it, by calling it, or just by mentioning it -- the two are coupled. By definition a system needs some coupling between its parts; otherwise it is no more than a set of disconnected parts rather than a system. But too much coupling and a system becomes hard to understand, hard to integrate, hard to extend, hard to fix, hard to test. In short, it becomes hard. If software is supposed to be soft, dependency management is one of the things that will allow it to be so.
Away from the vibrant discussion of the blogosphere and the considered advice found in many articles and books, the assumption that developers follow (or are even aware of) various best practices and discussions about design techniques does not hold as often as the well-informed developer crowd might believe, wish, or wish to believe. A lot of code is still written in monolithic style, with logic lumped together in roughly hewn, imposing slabs of code.
Such problems of code quality are often addressed by partitioning code into smaller units. There are many ways to break up and organize the general big picture of the system. But inevitably, in making any separations, there is also a need to make connections. As Edsger Dijkstra observed, "Separation of tasks is a good thing; on the other hand, we have to tie the loose ends together again!"
For example, from a deployment perspective, dynamic link libraries (DLL) offer distinct units of execution. Although they are discrete, they still have usage dependencies on one another. From a development perspective, one of the most common ways to partition a system is with respect to classes of objects. Again, although they are nameable and distinct, classes must still be related — by inheritance, association, or instantiation — otherwise nothing much is going to happen. The same applies to data in databases: In the relational model, data is partitioned across rows in tables. But the tables are not just buckets of arbitrary data (or at least they're not supposed to be); they are related through keys.
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