There are different ways in Astrology and each has its own importance. To decide a particular planet as benefic or malefic we use Shodasa Vargas (16 vargas) and shadbala (6 types of strengths). After deciding that a particular planet is either benefic or malefic, we have to see on which houses this planet has its influence.
A particular planet can have its influence on the house where he is posited, where he is aspecting and on the house he owns.
In the case of Sun and Moon, they will have influence on one house that they own, one house where they are posited and one house where they have aspect. (Total 3 houses)
In the case of Venus and Mercury, they will have influence on two houses that they own, one house where they are posited and one house where they have aspect. (Total 4 houses)
In the case of outer planets, i.e., Saturn, Jupiter and Mars, they will have influence on two houses that they own, one house where they are posited and three houses where they have aspect. (Total 6 houses).
That means a planet can have influence on maximum six houses in a particular horoscope.
There will not be any difficulty to an astrologer to predict results regarding these houses (at the most 6 houses) because the influence of the planet on these houses can easily be studied. But there are two ways of telling astrological predictions.
Immediately after seeing the horoscope the astrologer will have some ideas regarding the events of the native. There will not be any difficulty to tell those predictions to the native. But answering to the queries raised by the native is very important. If that quarry is pertaining to those houses on which a particular planet has direct influence as mentioned above, we can think about it basing on the influence of the planet. If the query is pertaining to those houses on which a particular planet has no direct influence (i.e., houses other than mentioned above) then what is the relation between that particular planet and particular house related to the query? How does this planet show its influence on the result of this particular query? This is a very delicate issue.
We can decide the benefic influence or malefic influence of a planet on the houses related to the query. But the query is related to the houses other than the houses directly influenced by the planet one has to depend upon Ashtakavarga only.
That is why, Pruthuyasassu, in his book HORASARA says
“ASHTAKAVARGAM VINAA DASAA PHALAM NA VAKTAVYAM ”
Seer Bhaguni says
“SUKSHMAA ASHTAKAVARGA SUDDHIHI, STHULAA SUDDHISTHU GOCHARE ”
A decision in broad way can be had from transit and a decision in finer way can only be had from Ashtakavarga.
In some other circumstances it is said that
“ VAIDHAVYA KANYAA KSHAYA HETUYOGE
KURVASHTA VARGASYA VADETTU SUDDHIM ”
Even though there are bad combinations like VAIDHAVYA (WIDOWHOOD) and KANYA KSHAYA (loss of girls) in the horoscope, these yogas will not give bad results if Jupiter’s Ashtakavarga is strong. Hence it is clear that Ashtakavarga is an important way of study of horoscope.