The classic Unix utilities are, traditionally, designed to do one or two things well, and fit together with other utilities to solve problems. This leads to a property known as composability, which is another way of saying that the utilities are small pieces which can be put together and rearranged in many different ways.
The present-day GNU versions of the utilities (and lots of the other programs that have accumulated in the era of Linux) relax the only do one thing idea, out of pragmatism and simple feature creep, but the basic idea holds.
Along with text files and the filesystem, standard streams, pipes, and redirection provide the necessary plumbing of a composable system. Pipelines can be written to explore and solve a huge range of problems by connecting simple operations together, and most tools can share data with other tools.
There's other shell magic to be learned, but the most important pieces are in place.
Next up, we look at aliases, wildcards, and the basics of treating the shell as a full-featured scripting language: An Illustrated Guide to Shell Magic: Typing Less & Doing More.