It is believed that, if the birth-chart is body of Kaalpurush then the Navamsa chart is its soul. The accurate prediction is incomplete without each other. The results of a planetary positions cannot be determined without the birth-chart while the strength of the planet which are in the malefic or benefic position in the ascendant chart can be cross checked with the position of the planets in the Navamsa chart.
As per the ancient scholars and astrologers, if the planet is weak in the birth chart and is strong in the Navamsa then the planet is not weak. In the same way if the planet is inauspicious in the birth chart and is auspicious in the Navamsa chart then the planet will be considered as auspicious. If the conjunction of ascendant lord of birth-chart and Navamsa chart is auspicious, then the person will be fortunate.
The concept of Vargottam is also judged with the help of Navamsa. Vargottam means if the planetary position in a zodiac sign in the birth chart and in the Navamsa chart is same. Then, that planet is Vargottam (eg. if the Sun is placed in the Aries in birth chart and in Navamsa also, then the Sun is placed in Aries). This placement makes the planet further stronger.