The devotees especially see the Lord in the beautiful blackish form of Śyāmasundara. That is the perfection of yoga. This yoga system should be continued until the mind does not vacillate for a moment. Oṁ tad viṣṇoḥ paramaṁ padaṁ sadā paśyanti sūrayaḥ: the form of Viṣṇu is the highest individuality and is always visible to sages and saintly persons.

The same purpose is served when a devotee worships the form of the Lord in the temple. There is no difference between devotionalservice in the temple and meditation on the form of the Lord, since the form of the Lord is the same whether He appears within the mind or in some concrete element. There are eight kinds of forms recommended for the devotees to see. The forms may be made out of sand, clay, wood or stone, they may be contemplated within the mind or made of jewels, metal or painted colors, but all the forms are of the same value. It is not that one who meditates on the form within the mind sees differently from one who worships the form in the temple. The Supreme Personality of Godhead is absolute, and there is therefore no difference between the two. The impersonalists, who desire to disregard the eternal form of the Lord, imagine some round figure. They especially prefer the oṁkāra, which also has form. In Bhagavad-gītā it is stated that oṁkāra is the letter form of the Lord. Similarly, there are statue forms and painting forms of the Lord.
(Srimad Bhagavatam----3:28:18----purport).

Lord Viṣṇu was personally present at the sacrificial arena, but this does not mean that He had any interest in His own personal benefit. Similarly, the arcā-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-ānanda-vigraha (Bs. 5.1) 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 Mahārāja Nābhi.
(Srimad Bhagavatam----5:3:9----purport).

Śrī Caitanya Mahāprabhu set forth three duties for three different people. Mukunda was to earn money and follow the religious principles, whereas Narahari was to remain with the Lord's devotees, and Raghunandana was to engage in the Lord's service in the temple. Thus one person worships in the temple, another earns money honestly by executing his professional duty, and yet another preaches Kṛṣṇa consciousness with the devotees. Apparently these three types of service appear separate, but actually they are not. When Kṛṣṇa or Śrī Caitanya Mahāprabhu is the center, everyone can engage in different activities for the service of the Lord. That is the verdict of Śrī Caitanya Mahāprabhu.
(Sri Caitanya Caritamrta----2:15:132----purport).

If the devotee knows perfectly that the arcā-vigraha, or Deity form of Lord Kṛṣṇa in the temple, is exactly the same sac-cid-ānanda-vigraha (Bs. 5.1) as Kṛṣṇa Himself, then his service to the temple Deity becomes direct service to the Supreme Personality of Godhead. Similarly, the temple itself, the temple paraphernalia and the food offered to the Deity are also not separate from Kṛṣṇa. One has to follow the rules and regulations prescribed by the ācāryas, and thus, under superior guidance, Kṛṣṇa-realization is fully possible, even in this material existence.
(Krsna Book).