tvam ekah sarva-jagata
tam tvam arcanti kusalah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
O lord, you are the cause of bondage and liberation of the entire universe because you are its ruler. Those who are advanced in spiritual consciousness surrender unto you, and therefore you are the cause of mitigating their distresses, and you are also the cause of their liberation. We therefore worship Your Lordship.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Actually Lord Visnu maintains and accomplishes all good fortune. If one has to take shelter of Lord Visnu, why should the demigods take shelter of Lord Siva? They did so because Lord Visnu acts through Lord Siva in the creation of the material world. Lord Siva acts on behalf of Lord Visnu. When the Lord says in Bhagavad-gita (14.4) that He is the father of all living entities (aham bija-pradah pita), this refers to actions performed by Lord Visnu through Lord Siva. Lord Visnu is always unattached to material activities, and when material activities are to be performed, Lord Visnu performs them through Lord Siva. Lord Siva is therefore worshiped on the level of Lord Visnu. When Lord Visnu is untouched by the external energy He is Lord Visnu, but when He is in touch with the external energy, He appears in His feature as Lord Siva.