William Arkle believed in reincarnation, and a means of Men gaining experience and being-educated across multiple lives. By contrast, angels are a separate creation, who gain their experience and education by moving 'downward' from spiritual Heaven deeper into the material realm of incarnation and earth - the job being both to help and educate Men and themselves to learn about the problems of imperfection and evil.
By contrast, my understanding is derived from Mormonism - which is that there is a three stage progression from pre-mortal life, as spirit angels, through incarnate mortal life ad via death to post-mortal resurrected life; this time as incarnated angels.
Yet, brooding on Arkle's understanding of the nature and role of angels and Men, which can be found in his works Letter from a Father and Equations of Being; I have realised that these provide considerable insight when interpreted from his scheme.
Arkle's angels correspond to pre-mortal existence; and he emphasises that the innocence, bliss and purity of this life is a deficiency of understanding - angels have no spontaneous understanding of the constraints of incarnation, mortality, and the evil effects and suffering resulting from free agency.
Therefore, while spirit angels work to educate and assist Men (when such interventions are of benefit - given that our purpose in mortality is primarily to learn for ourselves, by trial and error); the angels are of limited knowledge, and prone to make errors due to their lack of understanding. In fact, angelic errors are themselves an accidental but inevitable contribution to the evil and suffering of mortal life.
We might imagine a ladder from the spirituality of highest Heaven to the materiality of earthly-mortal existence; angels are descending that ladder, Men are ascending it; angels are the teachers, but also learning - Men are the learners, but indirectly acting as teachers of angels; both angels and men benefiting from the interactions.
Spirit angels existed before the first Men were incarnated as mortals, and have always been involved in earthly life; but we can assume that they will have found mortal problems both confusing and appalling - and they needed to learn from the experience.
We can imagine that - over time - more and more spirit angels will have learnt enough to recognise that they would benefit (in terms of progression towards full divinity) from voluntary incarnation as mortals; and then do this.
Over time, from the first mortal Men, there would be a development of angelic expertise, and eventually spirit angels were supplemented by incarnate angels who had experienced mortality.
yet, over this timescale, there will have been an accumulation of the effects of evil - so the problems of mortal life have also accumulated.
And the evil of mortal life has also been increased by the activities of fallen spirit angels - I mean demons. These demons perhaps include individual spirit angels whose interaction with mortal Men have led to various responses such as hatred, resentment, fear and the desire to dominate mortals.
For example, the prime demon - The Devil, Satan or Lucifer - is depicted in Mormon scripture as having rejected the divine plan for free agency in Men; and having fallen in order to destroy Men's free will, and to enforce a compulsory plan on Men (and other demons). The devil is therefore the prototypical totalitarian dictator; who believes he 'knows best' what is best for Men.
Arkle also assumes that the difficulties of mortal life, the accumulation of errors, evils and demonic power will eventually make mortal life just too difficult for the need for learning by experiencing; and this world will need to be ended, and another begun. In other words there will be an end time terminated by the end of this world (i.e. equivalent to Christ's second coming, the New Jerusalem).
Anyway, I find that this understanding of spirit angels descending and Men ascending and both interacting - to be helpful in making sense of what has happened in this world since the original incarnation of Adam and Eve; and the ways in which the problems and tasks of mortal life have changed throughout history.