The
Purpose of Purification
is Release:

Buddha explained that it is for the sake of
Nibbāna
that one trains the mind:
Morality is for the sake of self-control,
self-control is for the sake of no
regrets;
Absence of regrets is for the sake of joy, joy is for the for the
sake of bliss;
Bliss is for the sake of tranquillity,
tranquillity is for the sake of happiness;
Happiness is
for the sake of absorption into one-pointed concentration;
Concentration is for the sake of knowledge and vision of
how reality comes to be;
Knowledge and vision of how reality comes into being is for the sake of disgust;
Disgust is for the sake of disillusion, dry disillusion
is for the sake of relinquishment;
Mental
relinquishment; is for the sake of knowledge and vision of
awakening release;
Knowledge and vision of release is for the sake of
Nibbāna without
any clinging left!
For this purpose is this speech, for this purpose is this noble advice!
For this purpose should one pay attention to this chain of causes!
That is: For the purpose of release of mind through non-clinging...
Source:
Parivāra XII.2, SN II 32, AN V 2

More on Mental Release
(Vimokkha,
Vimutti):
Freed by Knowing,
Fulfilling_Release_by_Wisdom,
The_8_Deliverances