The reason is that the D1 chart and further divisional charts are very tightly interlinked and interconnected.

Using the most simple statement we hear about D1 and D9, “D1 is the tree and D9 is the fruit”, let us think about this:

Imagine 2 charts where D9 combination is exactly the same, but D1 combination is different. This is possible because planets can go into a particular Rashi in D9 from many different Rashis of D1. Example: a planet can go into D9 of Aries from the D1 Rashi of Aries, Taurus, Gemini, Leo, Virgo, Libra, Sagittarius, Capricorn, and Aquarius.

Now the manifestation of the karma for this person will be totally different.

For example, Mars going from D1 Virgo to D9 Aries is moving 8th from itself – results manifest via surgery, death, etc. Same Mars going from D1 Leo to D9 Aries is moving 9th from its position – results manifest via father etc. Similarly for all other planets. D9 May tell you a certain “fruit” is seen, but how will that fruit appear if there is no “tree” in the first place in D1.

Another simple example would be example for D9 lagna. Imagine D9 lagna is Cancer.
If D1 lagna is Scorpio – then D9 lagna is mapping to 9th house – mostly a good combination. If D1 lagna is Sagittarius- then D9 lagna is mapping to 8th house – mostly a difficult combination.

Divisional chart is born out of D1. It exists because D1 exists. If it is seen in isolation, then certain predictions can be made which will not go wrong, but more serious predictions and timings will go wrong – and there could be a difference of heaven and earth.

Now this applies to every divisional chart, every planet, lagna, etc.

Hence considering D1 first becomes important.