The necessity or reason of craving

[auto]

As to the alchemical agent, if the practiser is unmindful of it when gathering it, it is the prenatal microcosmic agent,
but if he is mindful of it it will be an illusory agent which leads to failure.

Craving is the cause of dukkha and reason for the sense consciousness to arise to resolve craving. If stomach empty then when seeing a cheese then consciousness of it arises. Whereas this type of resolving will happen again and again.

I think the above quote refers to the times when the craving is dormant then it is prenatal microcosmic agent what is gathered, whereas when there is craving; it is illusory agent, hence the sense-organ-consciousness what arises, perhaps it is in other words the wrong awareness/wrong light - becoming aware because of craving induced dukkha.
Yet it is thanks to the craving we come aware and could discern the wrong light to be wrong light thus attain right view - that there is kamma and supernatural
In short: Illusory agent = sense organ consciousness.