Shani , Rahu , Ketu & Surya bhagwan gives the astrologer a strong idea on what could have happened on ones previous birth. Surya being the atmakaraka (https://jyotisa.wordpress.com/2013/12/04/what-is-an-atma-karaka/) shows in which house(bhava) your ego is. This house represents who you were(past life) and the lessons that’ll be learned in this birth. Shani, Rahu & Ketu positions in the chart also shows what one has done previously which eventually made them to be born again. (https://www.youtube.com/watch?v=WxWq4IB4S8I)
Vedic astrology provides the reason why things are the way they are. The concept of ‘free will’ is a result of the impression that we carry from our previous janma (birth). This impression is called Jeevatma.
Om Tat Sat