It is my primary assumption that death is necessary for spiritual progression toward a higher and fuller state of divinity - but how does this work?
If the purpose of mortal life is incarnation (to get a body) then why? One answer may be that the body brings us irresistibly into contact with 'the world' - because the body is unavoidably part of the world.
(Whereas, as pre-mortal spirits, we were not in direct contact with the world, and our spiritual body - if we had one - was not subject to the world.)
So long as we are awake, alert and purposive in thinking - our body is spontaneously and by-default part of reality. And our consciousness (our spirit) identifies with the body (including the body's senses).
We spontaneously get-away-from the body during sleep - and especially in dreaming sleep when we are all-but cut-off-from the senses, and paralysed; in dreams we remain influenced by the body (e.g. illness can influence dreams) but we are no longer constrained by the body (we can dream anything, and be anywhere).
Much mysticism is trying to get away from the body - but insofar as this includes drowsiness or sleep, undirected or free-associative thinking, thinking directed by external cues etc. then this represents (merely) a regression towards the situation of pre-mortal spirit existence - which is fine, but regression is not the reason why we became incarnate mortals (otherwise we would simply have remained in our pre-mortal state).
Minimally we are incarnated for our spirit/ consciousness to identify with the body - and then die (this applies, for instance, to those people who die in the womb, or around the time of birth - they experience little more than the bare fact of incarnation).
Beyond this minimum, some people survive into childhood, adulthood - perhaps for many decades... what is that for? The purpose of prolonged life seem to be spiritual progression, divinization, theosis, sanctification (variously conceptualized) during mortal life.
I think this divinization necessarily entails a development, or evolution, of consciousness - of a type which is a foretaste of the post-mortal resurrected state.
This state of advanced consciousness has been variously termed a particular type of clairvoyance by Rudolf Steiner (also called Final Participation by Owen Barfield), or 'self-remembering' by Gurdjieff or Colin Wilson), or Active Imagination by Jung, or some versions of Abraham Maslow's Peak Experiences.
In essence, this is an awareness of ourselves as spiritual beings simultaneous with identification with the body and its senses.
Death is therefore the separation of spirit from body, the spirit having experienced the identification of spirit with body.
So there are three stages in spiritual progression in relation to mortality:
1. The spirit is identified with the body (mortal life)
2. The spirit separated from the body (death)
3. The spirit rejoined with the body from which it had separated
This three-stage process of incarnation, death and resurrection was - of course - established and made possible for us by Jesus Christ.
It leads to the post-mortal state of a permanent and habitual attainment of that state of consciousness which is typically only glimpsed (if experienced at all) during mortal life.
This is the divine mode of consciousness - which can then embark on further development.