Differences are supposed to convey information. E.g. a paragraph's first line is indented to indicate that this is a start of a paragraph. Differences in line width convey no useful information, so the are suppressed. Same goes for overall text height on a page, although this rule is broken more often.

Typesetters are known to be very specific about small details. E. g. tables are typeset with meticulous alignment rules. It may look superfluous, but it is not. The nature of the craft is that it has to be invisible unless you know where to look.

I've never given this much thought, but I agree with you 100%.

After reading this article I thought "wow that's really impressive and I hate it".

For the same reasons you mention, and the words seem to "blend together" into a blob of text and losing my interest.

Still super impressive that people do this though.

It's just neatness. Like ironing clothes or putting on a tie, it's not about it serving a functional purpose. It's just to show off straight lines.

Same here, justified text feels difficult to read. And, I had academia where where students are forced to justify text....