Could you please give me an overview of what David Malouf's poem "Towards Midnight" is about and its intended purpose?

This poem by David Malouf is broken into three parts: "The Cup," "Towards Midnight," and "The Rapture." The poem has the dedication, "for Joan Tesei (1934–2005)". A quick Google search tells us that Joan Tesei is a gardener in Tuscany, Italy, where many of the poems in the collection Earth Hour are set.
"The Cup"
This first section is, literally, about waking up. The speaker mentions drinking "heady mouthfuls / of breath" and the body reassembling "what sleep for a time / has scattered". The final stanza of this section refers to "Eternal Return." The capital letters of this phrase are a hint that they are referring to a set phrase.
Eternal Return, in philosophy, is the idea that all energy and existence will recur an infinite amount of times. The term is also used to refer to the existence outside of time when mythological events occur. This second meaning is sometimes likened to the Indigenous Australian concept of the Dreamtime.
Given that this poem is about waking up, the speaker is talking about the dreams and stories that "shadow- / like we carry in us. Sometimes / lightly. Sometimes not." Thus, the speaker is talking not only about literal dreams that we remember but also the archetypes and mythologies that we carry. It's also worth considering that perhaps the waking up and the new breaths refer to a newborn baby: the event of a soul being reborn.
"Towards Midnight"
The second section of the poem repeats the title. The phrase "towards midnight" (or "[x] minutes to midnight") is often used to refer to the doomsday clock used by the Bulletin of the Atomic Scientists to indicate how close humanity is to "Armageddon." The Armageddon is said to happen once the doomsday clock strikes midnight (linked below).
The imagery in this part of the poem is dark: a stranger, an "upstart a angel," "lurks" at the "margin of a room." There is "unease"—a "killing word we dare not speak"—and an unnamed "guest we have set a plate for, late to the feast." This section is about death and destruction.
The figure could be likened to the idea of the character "Death" who comes to take people away to the afterlife (present in many mythologies around the world) or an amalgamation of the horsemen of the apocalypse from Christian mythology. This ominous guest seems to be an ever-present, threatening presence for the speaker.
"The Rapture"
The phrase "The Rapture" refers to the judgement day in Christian mythology which happens at the "end of time." This section is full of the imagery of a Christian rapture: "being seized and taken," "lightening" (a play on the homophones lightning/lightening: both the weather phenomenon and your body becoming light as you ascend from earth), and:

the skyoverhead, suddenly trembledand crackedopen

Additionally, there is "the advent / of wings" and the image of being "swept up and / taken." Also consider the other meaning of the word "rapture," which refers to intense joy and being "carried away" by intensity of feeling (linked to the dictionary definition below).
Because of the strong imagery of leaving the physical world (as well as the allusion to the experience of intense and overwhelming emotion and given the references in the other parts of the poem to sleep and dreams), this section of the poem could be interpreted as being about the moment you fall asleep. When you're entering unconsciousness, you often have a feeling of weightlessness; dreaming is a purely psychological and cognitive activity—without the burden of your physical body. So the "loss of / gravity" that the speaker refers to has a double meaning: death (and ascension) and dreaming.
The intention of this poem is, in my opinion, to highlight the way that humans face our own mortality daily: in our dreams and subconscious and also in the mythology that we have created
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https://www.merriam-webster.com/dictionary/rapture