To remind us that God is everywhere, we may install a small temple in our home. If we have actually realized the presence of God everywhere, there is no need for temples, but most of us are not on that platform of spiritual realization.
So we need to be reminded constantly of the presence of God in our lives, and that is effectively done by worshiping Him in His deity form in the temple.
To engage us in His service God has kindly manifested Himself in the material world in a tangible form, in which we can practice our service to Him. We can bathe the deity, dress the deity, feed the deity, and in this way relate to God in loving, devotional service.
By such service, we’ll become gradually purified from selfishness. In the final stage we’ll be elevated to the uttamah
level of God-realization, the highest level of God-consciousness, where we actually see God everywhere.
Lord Krsna states in the Upanishads:
"One who worships Me in My Deity form while living in the district of Mathura or, indeed, anywhere in Jambudvipa, becomes most dear to Me in this world."
—Gopala-tapani Upanisad (Uttara 47)