Rituals and external worship can only bring benefit when they are coupled with inner spiritual feeling; external without the internal is meaningless.

(from "Kashmir Shaivism - The Central Philosophy Of Tantrism" by Kamalakara Mishra):

Abhinavagupta points out a very significant thing regarding extenal worship. He makes it clear that the external worship in itself has no power. It derives its power from the inner spiritual feeling that we attach to it:

vīryaṁ vinā yathā ṣaṇṭhastasyāpyastyatha vā balam |

mṛtadeha iveyaṁ syād bāhyāntaḥ parikalpanā || (Tantraloka 5.158)

"Just as without virility a man is impotent, and without life the body is dead, so is the external worship (without the inner spiritual feeling)." 

It is quite understandable that mechanically performed worship and rituals are meaningless. The recitation of mantra becomes efficacious only when the mantra is connected with consciousness. It is said,  

caitanyarahitāḥ mantrāḥ proktā varṇāstu kevalam |

phalaṁ naiva prayacchanti lakṣakoṭijapādapi || (Kularnava Tantra 15.61)

"Without consciousness (caitanya), the mantras are mere letters; they do not fructify even after being recited a million times.”

The power or life of the external forms of worship comes from the inner spirit or consciousness. The Mālinīvijaya Tantra says,

yajedādhyātmikaṁ liṅgaṁ yatra līnaṁ carācaram |

bahirliṅgasya liṅgatvam anenādhiṣṭhitaṁ yataḥ || (18.42)

"One should worship the spiritual linga within which the entire world is contained; the power of the external linga is based on this.”

But this does not mean that external worship and ritualistic practice (sādhanā) are useless. All it means is that the external derives its power from the internal, and that the external without the internal is meaningless. The observance of rituals and the worship of symbols, such as idols, have great meaning. If they are coupled with spiritual feeling, they not only become powerful, but they in turn make the spiritual feeling much more powerful. It is like the digits in the number 10. The 0 is valueless without the digit 1 before it. If the 1 is absent, the 0 is just a cipher. But when the 1 is placed in front of it, the 0 derives its value from the 1 and in turn makes the 1 ten times bigger. If the inner spiritual feeling is coupled with rituals and external worship, it is greatly intensified. This is the special significance of the rituals and external worship.

It is obvious that the external objects by themselves cannot purify the heart, but they can do so when they are combined with inner devotion or spiritual consciousness. For example, when one bathes in the Ganges river taking it to be just water, the physical river will not cleanse one’s heart. But if one considers the Ganges to be holy water — “fluid Brahman” — and bathes with this feeling of reverence, then the Ganges will surely cleanse one’s heart. What will really cleanse one’s heart is not the water but the feeling of reverence that one projects on the water. The same thing happens when one bows down in reverence to some external object — a deity, guru, parent, or anything at all. What helps one is not the external object but one’s own feeling of reverence and homage, which weakens the ego. But we should also not forget that the external object serves as a powerful medium for the projection of one’s feeling. In this sense the external object has a very significant role to play.

When one accepts a particular form of the deity, it is not that the particular deity itself is God; God is inside as one’s own higher Self, and while worshipping the deity, one unconsciously projects on it the God within oneself. Thus the worship of every external deity is an indirect worship of God as Self. Conceiving and installing the deity outside is therefore quite useful and sometimes even necessary, as it is difficult for people to worship the God inside them directly. For the very same reason we conceive of God as transcendent or the “holy other.” Since we are ordinarily not able to see the immanent God as our own Self, we have to see God outside as the “holy other.” Thus Tantrism comprehends within itself any kind of theism and any form of external divinization. But while worshipping God externally in a particular form, one must at the same time be philosophically aware that what one is worshipping is not itself

God, but a projection or a symbol of the real God, which is one’s own higher Self. The Tantric attitude saves us from falling into bigotry, idolatry, and superstition. In this sense Tantrism is the most rational religion.