Ravana was an ardent devotee of Shiva and there are numerous stories associated with Shiva, Shivling and Ravana. In the Uttara Khanda of the Ramayana it is mentioned that Ravana always carried a golden Shivling. Ravan had also composed the famous Shiva Tandava Stotra.
Where Ravana went he carried with him the golden Shivling for his worship. He used to create a pedestal of sand and place the Shivlingam on it. He then used to worship it. Puja was performed with incense and fragrant flowers.
Golden Shivling is intended for constant worship. Ravana worshipped it from the desire of sovereignty.
It is described in the Tantric texts that a golden Shivling should be worshipped when any one desires sovereignty.
It is also believed that worshipping a Shivling made of gold grants wealth (Sripadam).
Source – Elements of Hindu Iconography – Vol II. Part 1 – by T. A. Gopinatha Rao.