Since ancient times Navagraha Puja has been a Vedic ritual for bestowing health, wealth, education, domestic peace and prosperity. Worshipping the nine planets viz., Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, Rahu, and Ketu is a form of collective worship of the nine planets as per Vedic tenets. These nine planets have lot to bear on what one achieves or fails to achieve and the fruits arising thereof from karmic activities.
Though this sounds like a metaphysical argument, there is also the scientific reason with about planets influencing us with their gravity and electromagnetism which can impact over long distances.
Every human life is therefore influenced and certain good or bad happenings result and affect various aspects of life such as love, marriage, career, finance, health. When planets work negatively in the horoscope they combine to form good & bad yogas.
All that is required is puja be done with devotion and sincerity of purpose adhering to rituals as specified. This brings peace and success in life. Navagraha Puja has to be done on a special day (muhurat) taking the horoscope of the native into account. In Navagraha homam mantras are chanted for all the nine planets for Graha Shanti (propitiation of planets) thereby bringing happiness, prosperity, winning over enemies, wealth and many more such benefits.