That would really depend on the religion. In Buddhism, ritual is used to both express and reinforce attention, compassion and mindfulness.

Ritual to me means "repetitive practice". With repetitive practice, an action becomes automatic. You no long have to pause or stop to think about what you're doing. It's been said that thought or the intellect can only take you so far with religious practice. In Zen, thought is described as "a finger pointing to the moon". Mistaking the finger for the moon is like confusing religious BELIEF for religious EXPERIENCE (spirituality). Thought/ego often gets in the way of actual experience, therefore techniques or rituals are used to gently coax the mind away from thought/ego toward actual experience.

Hinduism: A very important function of rituals is determined by its symbolism. Most of the rites we perform are intended for visualizing belief. Our gratitude to God is visualized by our offerings of flowers and other things on as altar. Our humble service to Him is visualized in the temple where He is hreated as an earthly king and highest honors are paid to Hi. Rituals have obvious psychological function of providing an outlet for religious emotion. Like every emotion tries to seek an outlet in action it finds satisfaction in something that is done under its influence so is the religious emotion craves for expression and finds it in the prescribed ritual. Rituals have also a moral function. In fact, we may say that rituals in one sense are incipient morals. Almost every rite we perform is a lesson in self-control. It is the first step in that long process of the liberation of spirit from the thralldom of the flesh. The most important subjective function of the rituals is what may be called its mystical function. Soul feel the mysterious presence of God. All the Gods and Goddesses are invited with particualr Mantras. There are basically two kinds of Mantras for inviting and worshipping the Gods and Goddesses. 1. Vedic - which have been used since the time of Veda. 2. Tantrik - which bears the seed to provoke certain things, with the intention that the PUJA is being performed. http://www.godmandir.com/rit.htm A detailed series of life-cycle rituals (samskara , or refinements) mark major transitions in the life of the individual. Especially orthodox Hindu families may invite Brahman priests to their homes to officiate at these rituals, complete with sacred fire and recitations of mantras. Most of these rituals, however, do not occur in the presence of such priests, and among many groups who do not revere the Vedas or respect Brahmans, there may be other officiants or variations in the rites. A crucial event in the life of the orthodox, upper-caste Hindu male is an initiation (upanayana) ceremony, which takes place for some young males between the ages of six and twelve to mark the transition to awareness and adult religious responsibilities. At the ceremony itself, the family priest invests the boy with a sacred thread to be worn always over the left shoulder, and the parents instruct him in pronouncing the Gayatri Mantra. The initiation ceremony is seen as a new birth; those groups entitled to wear the sacred thread are called the twice-born. http://atheism.about.com/od/hindusandhinduism/a/IndiaRituals.htm

I don't personally think that they serve any purpose other than to reinforce to people the gravity of thier particular religion. To me belief in a deity shouldn't involve rituals..it should be a personal thing.