The LaTeX typesetting system has become famous because of the high quality rendering of text. What most newbies doesn't know is that the latex community is using the term typography with a certain bias. The assumption is that good typography is equal to justified text. Justification means the opposite of flush left. It means, that the right edge is straight from top to bottom.
A short look into the history of mechanical book printing shows that the latex community is arguing with the same assumption like previous technicians. What the first gutenberg printing press, the linotype and the intertype photosetter have in common is the ability to produce fully justified paragraph. The goal of former typesetters wasn't only to print something on a paper but the goal was to print it with the fully justified layout.
It is a seldom described fact, but all the academic journals and newspapers in the last 200 were printed in the fully justified mode. This might be surprising because before the advent of desktop publishing only hot metal typesetting was available. That means there was a need for manual intervention to create a straight right edge.
It remains unclear why this unwritten rule in typography is valid for such along period. Perhaps it has to do with western tradition. The assumption is good typography is equal to justified text and this is the paradigm behind the latex software as well.
From a technical perspective all the printing device like the linotype machine and modern word processing software is able to create left flushed paragraph easily. In most cases the algorithm for doing so is much easier to realize. Because at the end of the line the last word gets wrapped to the next line and no in word adjustments are needed. But the interesting situation is that for final printings this ability is never used. Especially for academic publication there is an unwritten rule available that the text needs to be fully justified. The question is why?
Sometimes the argument is provided that a fully justified text can be read faster. Another argument is that the layout looks better because the columns can be recognized. But in most cases the real argument is that fully justified text has a long tradition and therefor new typesetting has to follow this rule too. Basically spoken it is not a need but it is personal preference why professional publishing is equal to fully justified formatting.
Since the internet age the situation has changed drastically. 99% of the web-rendered websites are using the flush left layout. All the browsers which includes lyx, firefox and the internet exploring are rendering text in this way. Similar to the situation with justfied text in a printed world, the main reason why all the browser are using flush-left rendering is because everybody else is doing so. That means in the internet world there is an unwriten rule that a paragraph needs to be displayed that way. The interesting fact is that the argument is the same like for the book printing world. The reason why firefox is using flush left rendering is because such a text can be read faster and it looks better.
The unsolved question is what is the best formatting style in an electronic pdf document? Nobody knows, a pdf document is using the us-letter format which is working different from a website. Technically it is possible to use justified and flush left formatting as well. So it is an individual decision how the typesetting is made. But according to the produced documents the amount of justified pdf papers is higher. A short look into the arxiv repository will show that 99% of the papers are fully justified. Especially the academic community beliefs that this unwritten rule is valid. A document formatted only with flush left is common for draft documents in which improvements are needed before the final publication.
Mass printing
What newspapers, books and academic journals have in common is that the amount of printed copies is high. A single issue of a newspaper gets printed many thousands times. This high circulation makes it economical useful to make the typography very accurate. It makes sense to use the Linotype printing machine to produce fully justified text. That means the human typesetter will need many hours until the layout is ready and nobody cares because many thousands people will read the result.
In contrast, manuscripts which are circulating with low amount of copies are created with simpler machines like a mechanical typewriter. A typewriter has no advanced featues like a fully justified text. Therefor normal manuscripts are flush left.
With the upraising of digital printing the situation has become softer. IN most programs it is only a simple click with the mouse to change the setting from justified to lush left. What we can say for sure is that in the past a justified text was equal to high amount of copies. Because a justified text was created by a professional typesetter and this can only be realized if a larger audience is interested in a book or journal.