The moment of death is so important because our next incarnation depends very much on how we live our last moments in this one. Our state of mind at this time affects us in the other world up until our rebirth.
You will have noticed that the state in which you go to sleep affects you during the night, and influences how you feel on waking the next morning.
Even when you have had a good day, if you go to bed in a negative state, you will wake the next morning in a bad mood.
On the other hand, even if your day was not too brilliant, if you go to sleep with positive thoughts and feelings, these last moments before sleep will fix and clear everything up, and you will wake the next day in better spirits.
These same laws operate from one life to the next.