If its saguna sakara brahman Narayana (Purusha) who creates and preserves everything in this universe, and Shiva (Purusha) who destroys everything, then what is the job of prakriti or shakti?
The question is:
what is the job of prakriti or shakti?
Sri Krishna says in BG as follows:
पिताऽहमस्य जगतो माता धाता पितामहः।
वेद्यं पवित्रमोंकार ऋक् साम यजुरेव च।।9.17।।
Of this world I am the father, mother, ordainer, (and the), grand-father; I am the knowable, the sancitifier, the syllable Om as also Rk, Sama and Yajus.
सर्वयोनिषु कौन्तेय मूर्तयः सम्भवन्ति याः।
तासां ब्रह्म महद्योनिरहं बीजप्रदः पिता।।14.4।।
O son of Kunti, whatever forms are born from all the wombs, of them the great-sustainer is the womb; I am the father who deposits the seed.
सत्त्वं रजस्तम इति गुणाः प्रकृतिसंभवाः।
निबध्नन्ति महाबाहो देहे देहिनमव्ययम्।।14.5।।
O mighty-armed one, the alities, viz sattva, rajas and tamas, born of Nature, being the immutable embodies being to the body.
The Almighty has 2 forms,ie., sAkAra and nirAkAra - with form and formless.
The sAkAra form of the Almighty is called prakriti or NATURE. Like a Mother it gives sustenance to all creatures.
I think I read something about it in later chapters on Gita, or in Mahabharat (Don't remember exactly).
It is something like, Krishna says that he created the universe and Prakriti, then gave the responsibility of running this world to Prakriti, then Prakriti created some rules to run the world, Now even Krishna follows those rules, he can choose to break them at his will, but does not.
If you give me some time, I will try to find the exact source of where I read it.