What is the necessity of worshiping idols in temples?A.
Idols or images of God are inert objects. A form carved in a stone or painted on paper is an imaginary form and is not even a direct photograph. Idols and images are only models representing certain concepts. They facilitate knowledge.
The form of idols and photos is mainly human. This represents the concept that the Lord always comes to this world in human form as is said in the Bhagavad Gita (Manusheem Tanu Mashritam...). Please remember that the Gita did not say that the Lord would come in any other form. The forms of fish, tortoise etc., which the Lord took, as mentioned by the Puranas were only temporary. They were meant only to kill some demons and nobody worshipped such forms even during their time. Rama, Krishna, etc were the human forms of the Lord. They were worshipped by several devotees like Hanuman and Gopikas in their time and are being worshipped even today.
The Lord will come in every human generation; otherwise, He would be accused of being partial to a particular generation or age. The Lord comes whenever there is a necessity as said in the Gita (Yada yadahi). Once this concept is realized, there is no need of going to the temple and worshipping idols One should go from school to college and then to university. This does not mean that when one leaves the school, the school should be destroyed. The school must exist for others who need it.
Similarly, if for one person the idols and the images of God are not necessary anymore, it does not mean that the idols, photos and temples should be destroyed. They should be protected and must be respected as the models of divine knowledge for the ignorant devotees who need them. Some devotees cannot accept the human form, which is before their eyes as said in the Veda (Pratyaksha dvishah). For such devotees, idols and photos are necessary for meditation since they are still at the high school level in spirituality. Idols and pictures are useful for meditation for such limited minds as said in the Sastras (Pratima svalpa buddhinam).
The Veda says that the Lord does not exist in inert objects (Natasya pratima, Nedamtat), but It says that inert objects can stand as models representing the Lord (Adityam brahmeti). Therefore seeing and meditating upon idols and photos are acceptable in the case of ignorant devotees. However, a lot of other rituals like offering food, burning camphor, fume sticks, oil lamps and breaking coconuts and offering flowers are not mentioned in the Vedas and are unnecessary. These unnecessary rituals should be avoided especially since many of them cause pollution and harm humanity. Offering food should be done to the human form of the Lord only, where it is befitting and not to the inert idols. Ijya or yajna is cooking and offering of the food to God. The Gita says that such ijya should not be done to inert objects.
Source - http://www.universal-spirituality.org/rituals.html