Why we need a Deity

He [the Lord] accepts the form of arca-vigraha (worshipable Deity) just to accept service from His different incapable devotees. By the mercy of the arca-vigraha, the form of the Lord in material elements, the devotees who are in the material world can easily approach the Lord, although He is not conceivable by the material senses.

(SB 1.12.9 - Purport)