Srimad Bhagavatham-5.3.9. :
O Lord of lords, we are completely ignorant of the execution of dharma, artha, kama and moksha, the process of liberation, because we do not actually know the goal of life. You have appeared personally before us like a person soliciting worship, but actually You are present here just so we can see You.
You have come out of Your abundant and causeless mercy in order to serve our purpose, our interest, and give us the benefit of Your personal glory called apavarga, liberation. You have come, although You are not properly worshiped by us due to our ignorance.
Lord Vishnu was personally present at the sacrificial arena, but this does not mean that He had any interest in His own personal benefit. Similarly, the arca-vigraha, the Deity in the temple, is present for the same purpose.
Out of His causeless mercy, the Supreme Personality of Godhead presents Himself before us so that we can see Him. Since we have no transcendental vision, we cannot see the spiritual sac-cid-ananda-vigraha of the Lord; therefore, out of His causeless mercy He comes in a form we can see.
We can only see material things like stone and wood, and therefore He accepts a form of stone and wood and thus accepts our service in the temple. This is an exhibition of the Lord's causeless mercy. Although He has no interest in such things, in order to receive our loving service, He agrees to act as He does.
We cannot actually offer suitable paraphernalia for the Lord's worship because we are completely ignorant. It was out of His causeless mercy that the Lord appeared in the sacrificial arena of Maharaja Nabhi.