It will tweak spacing, kerning, margin protrusion, and font size to improve readability avoid big word gaps and excessive end-of-line hyphenation.
It is what sets professional typography apart. Only Adobe InDesign provides a comparable implementation, tweaking all those details.
See https://en.wikipedia.org/wiki/Hz-program for a better explanation and an example.
IMHO, the difference is obvious and not minor. Without microtypography texts look ugly: https://upload.wikimedia.org/wikipedia/commons/0/03/Hz_Progr...
Sure, I don't like creeks like in your last example. But I absolutely prefer paragraphs, where the final line would be considered 'too short'. It also makes an appreciable impact for me, in how easy a text is to read.
Which is to say, half of these things are pretty subjective.
> Only Adobe InDesign provides a comparable implementation, tweaking all those details.
TeXmacs claims to have implemented microtypography as well (https://www.texmacs.org/tmweb/home/news.en.html, as I am reading it, in the opening paragraph on version 2.1)