The TeX typesetting system is one of the largest and important programs in the history of the Unix operating system. The first electronic generated documents ever were created in the last 1970s with the TeX software and until now all the scientific journals are created with this software only. From an abstract perspective it is difficult to understand why LaTeX is so popular. The simple reason is that it has the build in feature of creating justified text.
To understand why this feature is important we have to go back in the history of typesetting. The monotype mechanical typesetting machine was invented around 1900. It contains of two parts. The desk for creating the punched tape, and the casting machine which takes the punched tape as input and produces the hot metal matrices. The interesting situation is, that even the monotype machine was able to create fully justified text. Workers who have used the machines 100 years ago can confirm this statement and very important, newspapers and journals from that area were typeset in this format.
How can it be that a magazine 100 years ago was using fully justified text? Isn't it really complicated to stretch and compress a line of word this way? Yes it is complicated, but the monotype machine was using this feature all the time. It was the main difference between ordinary typewriter produced manuscripts and professional created journals.
The ability to typeset fully justified text was re implemented by the TeX software later in the 1970s. Similar to the monotype machine, the tex software is using this formatting style as default. The assumption is that the needed output of the typesetting process is always the fully justified text. This workflow is described as beautiful typesetting. In contrast text which is only left justified is seen as the opposite.
Why is justified text better than flush text? The main reason is perhaps because the first category is harder to realize. Creating a text in this way requires more effort than producing ordinary text. It seems that this extra effort has become a tradition.
Let us analyze the typesetting process around 1900. The input for the human typesetter was a manuscript created with a mechanical typewriter. Like all manuscripts such a document was using a flush left alignment in a single column mode. Because such an output is easier to create for a typewriter. The main task of a monotype operator is to convert this input document into a fully justified description on the punched tape. That means, the operator beautifies the manuscript by realizing the justified mode.
In summary the monotype machine from 1900 was able to create fully justified text in multicolumn layout. This combination was needed for academic journals, books and newspapers as well. That means justified text is not an invention from the computer age, but it was common in the time of mechanical typesetting.
In contrast, the widespread usage of left justified text is a symbol of the digital age. The internet was the first medium in the history which was using this formatting style everywhere. The reason was, that before the internet was invented no other public medium like printed journals or newspaper were using left justified text. That means, the justified text which is more complicated to realize was invented much earlier and the flush left text which is easier to realize is a new thing available since the 1992.