Hindus worship various gods and goddesses. Like Ganesha for success. Lakshmi for wealth. Durga for protection. Similarly, why is Shivalinga worshipped? As you mentioned yesterday, Shivalinga is a very powerful symbol of divine power working on earth. It is representative of the union of Shiva and Nature. So is Shivalinga worshiped to seek prosperity, nourishment, strength from Prakriti that is Parvati and wisdom, knowledge from Shiva🤔?

As to what one can ask is upto the one who is worshipping. As Sri Krishna says in the Gita that whatever god one worships and whatever boon one asks it ultimately reaches the Supreme Divine, Purushottama who is the ultimate giver and sanctioner of all boons. So a lot depends upon one’s faith. Shiva gives Moksha, that is freedom from ignorance. It is not any ordinary knowledge but a knowledge of the ultimate Reality that he brings to the seeking soul. Prakriti, on the other hand means all that the world can offer, worldly powers of various kinds. The combination itself means that one gets what the world can give but is not attached to it ignorantly due to the liberating knowledge of Shiva that governs Prakriti. 

Affectionately,

Alok Da