"Don't you know that you yourselves are God's temple and that God's Spirit lives in you?" 1 Corinthians 3:16
The English word Spirit, comes from the Latin spiritus meaning "breath"
Our breath is our life force. Its our most important external resource we need to internalize and also once again externalize. It is spherical in nature because breathing in could not happen if one did not breath out and vice versa. Breathing is the only function of the body that you can both control and have no control over. If unaware breathing takes place on its own, but with attention it can be lengthened or reduced to change the acidity of the blood. Proper breathing reduces stress, supplies oxygen via the blood to our stressed muscles and focus upon the breath develops self-awareness and concentration.
Meditation is a mental discipline where we bring ourselves to a calm and relaxed state by slowly reducing our thoughts so they become less reflexive and automatic. By bringing ourselves again and again to this calm state the brain stops becoming either too creative or too logical and starts to balance itself. When this balancing occurs the brain starts to produce waves of a higher range.
Brain waves range from Gamma(30-100 Hz) to Beta(12-30 Hz) to Alpha(8-12 Hz) to Theta(4-7 Hz) to Delta(up to 4 Hz). These have all been recorded in humans and some much more frequently in animals using an EEG(Electroencephalography) machine.
As one meditates because of the sensory deprevation of closed eyes and the relaxed body waves switch from Gamma waves which are common in cognitive or motor function to Beta waves which is common when was is active, busy, anxious, thinking or alert to Alpha waves which occurs when one is relaxed and reflecting(on one thought).
Theta waves unfortunitly dont occur often in adults but are much more common in children and when they do occur for adults they are in the moment of "Ah ha!" when you finally break through a problem a theta wave will appear and disappear just as quick as the words did. Theta also occur during the REM cycle of sleep where one is having the most vivid portion of the dream. It also occurs for moderately practiced meditators.
Delta waves on the other hand is much more rare and found only during adults during their slow wave sleep (where they heal and grow the most), in highly experienced meditators and in babies. But it is from these delta and alpha waves that the deepest and most powerful thoughts occur and when directed on proper topic great insights can occur. Plus maybe some spiritual stuff too ;).
The best way to reduce our thoughts is to focus on one thing or no thing instead of everything. Whereas to think upon one topic long enough brings about insight and understanding of the nature of the topic. Focusing on no thought brings about awareness to the nature of experience before our labeling. Unfocussed thought leaves our attention and concentration scattered and is like starting 100 different tasks none getting finished.
Because breath is our vital force and constant it seems natural that it is a focus point for meditation. Focussing on the breath is the most commonly known method of people reaching "enlightenment" or delta wave recorded. This was the method used by the Buddha and thousands of others.
There is a book called Vigyan Bhairav Tantra which is around 5000 years old dialoguing a conversation between the Hindu diety Shiva(God of Destruction) and Devi(goddess representing consciousness). In this dialogue, famously translated by Osho(a mystic and spiritual mentor) it describes over one hundred methods of spiritual realization via forms of meditation. The first and second method being variations on how to have awareness of breath. The translation can be found here.
Though spirituality is not the only nessisity of life as many may think, it is one of the most important parts as it leads to happiness, greater creativity, natural positive outlooks, more compassion, more awareness, more friends, more strengths and less weaknesses.