The Ngondro practice is very important for purifying negative karma and to generate wisdom. Actually, our main practice is Mahamudra, but you cannot practice Mahamudra without the purification or the blessing. In this context, the "Preliminary Practices" are most essential.
You are in samsara now, and as long as you remain thus, you will experience disturbing emotions. Otherwise, you would already be enlightened. In the past, no matter where you were born, you experienced various poisons of the mind. This is true regardless of whether you were in the higher realm of beings or whether you were born in the lower realms. Your present state is proof of that, because were it different, you would not be experiencing disturbing emotions now.
So long as these disturbing emotions are in you, you are accumulating negative karma. However, it is not as if a certain karma was accumulated, it would then ripen to give a certain result, and afterwards, that karma would disappear completely. And that the only karma left in us now is what has caused this present human life. No, you have millions of different karmic imprints built up from many past lives. In addition, you are continuing to create much new karma through your thoughts, speech and actions everyday. Sometimes they are positive, sometimes negative. But unfortunately, they are most often negative. As humans, we are constantly involved in disturbing emotions which can never result in anything positive.
This does not mean that we should belittle ourselves. Rather, we should accept our present situation - this is our karma now, and it is preventing wisdom from appearing. This wisdom is already there. It is the nature of our mind. However, our disturbing emotions cover it. From our disturbing emotions we create karma. The result for us is more samsara where we create more karma.
So our karma is very strong. We weaken it by doing the "Preliminary Practices" until it cannot harm us any more. We practice the accumulation of merit through the Mandala-Offering, the third of the Ngondro practices. This will create in us all the necessary conditions to reach enlightenment.
When we are freed of karmic influences and have accumulated all the positive conditions, we can successfully begin the Mahamudra practice. If, however, after 111,111 repetitions of each of the four preliminaries, we realize that no development has occurred, then we have to continue to work hard on the preliminaries in order to weaken the negative karma.
While practicing Ngondro, many good signs may appear. They are an indication that a result has been reached. But one should not have too many expectations regarding these signs. They should appear naturally as they cannot be artificially produced.
After having practiced Ngondro extensively, students receive the Mahamudra teachings. It would not be very beneficial to teach them the Mahamudra before that because they would not be able to understand them precisely. The mind must be purified for that to happen.