Regulus is the best available demonstration of why precession matters to this material, because it crossed a boundary within living memory.
The earth's axis wobbles with a period of about 25,800 years, which causes the equinoctial points to slide westward along the ecliptic at roughly 50 arcseconds a year, one degree in about 72 years. Tropical zodiacal signs are measured from the equinox, so the fixed stars drift steadily forward through them. This is not a theory; it was detected by Hipparchus in the second century BC, and he detected it using Spica.
The recorded position of Regulus in this collection is 29 degrees 50 minutes of tropical Leo, an epoch-2000 figure. That leaves ten arcminutes to the boundary of Virgo, which at one degree per seventy-two years is about twelve years. Regulus accordingly entered tropical Virgo around 2012, and it is there now. A star that had been the heart of the Lion, in the sign of the Lion, for the whole recorded history of Western astrology, is no longer in that sign.
Tropical and sidereal astrologers weigh this very differently, and the disagreement is genuine rather than a misunderstanding. Sidereal systems measure from the stars, so Regulus never moves relative to the sign and nothing has happened. Tropical systems measure from the equinox, so the star has changed signs and some tropical practitioners regard the ingress as significant while others treat fixed stars by conjunction to natal points and consider the sign irrelevant.
The general point is unavoidable, whatever one's position: a medieval attribution of a star to a degree of the zodiac refers to a different ecliptic longitude than the same star occupies now, by roughly fourteen degrees since Ptolemy.