The Unix operating system was originally created in the early 1970s. Apart from a number of technical innovations it made, or brought together, its develoment has been driven by an ethos often named "the Unix philosophy". While this wasn't explicitly written down by the original developers, there are various formulations. Perhaps the most succinct one is by Doug McIlroy:
This is the Unix philosophy: Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface.
Others have expanded on this, up to and including writing books on the topic. See the Wikipedia article for some discussion and references.
This philosophy is clearly present in, and clearly drove the development of, the older parts of Unix, such as the most basic command line utilities and how they are used together. For example, instead of having each program that produces output have an option to sort the output, the sorting is put into its own program, and the two programs can then be combined together into a pipeline.
The ethos has waned in later years, due to factors such as the move to graphical user interfaces and desktop environments, and the proliferation of the kinds of data we now deal with. GUI applications are not so easy to combine together as command line filters: it doesn't seem like anyone's designed a system where that works well. The kinds of data now processed are often not textual, and are instead graphics, audio, video, or structured documents using a markup language.
However if you use, or program for, a Unix system then it is good to be aware of the philosophy. It will help you understand why things are the way they are.