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...