Your question is really interesting one. To understand this you need to understand karimic level of energy passing. That is heritage. Our parents santana yoga influenced to child birth. It happened because of consumption. That was the influence of planets. Every object have certain energy to influence other. Example magnet and iron. it depends on its distance like that planets have it own size and influence. Our ancestors might done vast research on it. They might considered the key value is snap shot of birth time is base for it. For every calculation we need a base that is X value. That we can consider birth or prashna time or face reading or palmistry , nadi lot of methods to predict the future.