Retrograding planets are not actually moving backward in the sky. From the perspective of Earth because of differing orbits, the planets are in perspective moving backward across the zodiac belt.
Harmonically it brings up a re-view of whatever has just been experienced at those active zodiac degree marks.
Retrogrades support introspection because it’s like universal deja vu. Retrogrades are sacred in the sense that at this moment in time, these particular themes related to this portion of the zodiac have been chosen as themes to experience 3 times. (3 being a magic number too) First direct, then in retrograde, then direct again.
Retrogrades are perfect times to get into Sabian symbols also. I use this book by Dane Rudhyar, but there are plenty of sources you could look up online.