I have attached an image to understand the importance of this degree, which for example is present in the Prometheus Capricorn software. In the chart, we can observe Pluto, which at the time of observation has an extreme latitude (+8). In the chart, Pluto is indicated at 19? e 33' Leo (which is correct) in the 6th house, but its latitude is so extreme that in the local sphere, in optical reality, Pluto is in the 7th house and not in the 6th. In fact, the calculated Passage Degree is 6?06' Virgo, which identifies the "real" position of Pluto, real in relation to the perception of the local sphere.
It's a somewhat complicated concept to understand, but as you know, planets also have movement in latitude, and the zodiacal longitude is a value given without respect to latitude, as if the planet were always at latitude zero. The Passage degree (used, for example, by the Prometheus software) is a sort of indicator of the real position of the planet and corresponds to the position in DH.