Why one needs to show his chart to an expert or good astrologer?

There are many contradictory views existing in Vedic astrology. Contradictions exist in modern literature as well as classics on astrology. To sort out those contradictions an experts view is needed. The reason is that the expert has worked on many real charts & applied the theory in practical life too.

Let us take an example. How important is Sakata yoga? It is given in classics , but should it be blindly applied on charts or not?

Sakata yoga: If Moon is in 6th, 8th or 12th house from jupiter, then this yoga is formed. If Moon is in a Kendra then such a yoga is cancelled.

Result: Unhappy, unfortunate. Sometimes his luck dawns but other times it fades away.

Now this is the view of Phaldeepika & some other classics on SAKATA yoga. But I in fact don't use this yoga. I think that this yoga is not valid. If Moon is in 12th house from Jupiter , then Jupiter would be 2nd house from from Moon. This would make another yoga "Sunapha yoga" that would make someone a king, famous & rich. So Sakata yoga is directly in contradiction with Sunapha yoga.
Moreover Jupiter as a reference point in this yoga seems to be weird, as Chandra could be a better reference point.  

Hence I think that this form of Sakata yoga is an invalid yoga.

There is an alternate form of Sakata yoga given in Saravali. That yoga definitely seems to be true , but not this one.