Those who know the rituals to the Pitṛ know that a child, especially a son, is required to perform the rituals which will liberate the parents, especially the father, from the wrath of Yama and the Pitṛ.
Those who know the Indian planetary mythos know that Śani (Saturn) is the son of Sūrya (the Sun).
Those who know the astrological symbolism of the planets see that Saturn indeed liberates the Sun.
The Sun represents ātmā – the fundamental consciousness itself, the core of who and what we are. The Moon represents manas – a reflection of the light of consciousness into the dark night of insentient, external, material things. Mercury further extends the light so that it can interact with these insentient things, via buddhi (intellect, the i/o bridge between mind and matter). Venus grants five actual senses of perception to the intellect. Mars grants five actual senses of action.
However when there is action towards the objects of the senses, living beings come into conflict, and fight over the objects – Mars runs rampant.
Jupiter enters with morals, principles and values dictating how people must share and cooperate. Live becomes good.
But the living entity is still hyperextended into an environment foreign to itself, external to its true being. So, in spite of all the prosperity Jupiter can bring, there is still depression and dissatisfaction. There is still hollow loneliness.
Saturn enters to smack us on the face and wake us up. “See, everything you are striving for is foreign to you. You are permanent, but you are trying to enjoy situations that are temporary. Wake up! This will never work!”
Saturn allows us to change our concept of self, and thus encourages the sunlight which has evolved through the Moon, Mercury, Venus, Mars and Jupiter, to finally return within itself, to what it truly is – a purely conscious entity with joy and happiness within. This is something that cannot be realized without disconnecting from the falsehoods of joy that surround us.
So, symbolically speaking, Saturn liberates his father, the Sun, from his forray into pursuit of external objects, by forcing his father to realize his mistake, by forcing him to face the temporality and non-esstiality of all the things his father is obsessed with via the other five planets.
Thus Saturn is the ultimate mokṣa-kāraka of the planets, and the ultimate indicator of spirituality.
PS – Rahu and Ketu are the evolutionary forces themselves. Rahu allows the Sun to extend outwards through the Moon, up to Mars. Ketu allows the Sun to return inwards from Jupiter through Saturn. Thus Ketu is also a mokṣa-kāraka.