It has been ascertained that the earth and other objects are the created ones (they are basically products) and ascertained that the creator of all these products is Isvara. Since that Isvara is a Supreme Being associated with perpetual contentment (nitya trupta), the created objects could be of no use to Him. Nor the created worlds could utilize themselves, since they are non-intelligent. Since Lord Siva is with such a greatness and lordship, he cannot perform purposeless actions; He cannot create purposeless objects. Therefore, through the process of elimination, it is ascertained that the creative function and the created worlds are neither for their own sake nor for the ever-content Lord who has created all these, but only for an intelligent being existing apart from the created and the Creator. That intelligent being is known as the kshetrajñahah. Mregendra āgama 6.2.
Abhinavgupta provides following explanation about kshetrajñahah.
The field (kshetra) is the body of worldly beings where the seed of action is nurtured. In the case of worldly beings the embodied self (jivātma) is called the knower of the field (kshetrajñahah) , who remains polluted by temporary impurities. However, in the case of enlightened ones, the embodied self is called kshetra.
Therefore it is clearly explained, Lord Shiva has not created the universe for his own, he created for Kshetrajñahah. But why would Shiva create the universe for them?