Question: Why do Pure Land Buddhists chant [A] with the dying and [B] for the already deceased?
Answer: [A] When someone is dying, support-chanting is done for these two reasons:  Merits: To create merits for and with the person, so as to help eradicate negative karma – for lessening suffering while increasing blessings in their last moments for peaceful passing (rebirth).  Guidance: To guide the person to understand about Amituofo and his Pure Land, so as to have strong Faith in his power to guide him or her, to have firm Aspiration to reach Pure Land, and to have the crucial Practice (of Nianfo: mindfulness of Amituofo’s name) to do so.
The Three Provisions (Faith, Aspiration, Practice) for birth in Pure Land can be nurtured by summarising the Amitabha Sutra to the person, followed by chanting it, with chanting of Amituofo’s name (Nianfo) as the main Practice. Chanting of Amitabha Sutra is for introducing Amituofo and his Pure Land, while chanting of Amituofo’s name is for showing the practice needed. If time is running out, chanting of the sutra can be skipped, although there should still be summarising of the purpose of Nianfo before actual Nianfo. Without this understanding, Nianfo could turn out ineffective.
[B] When someone is already deceased, chanting is still continued as much as possible, especially within 8 hours of passing and as frequently as possible for 49 days for reasons similar to the above, as the consciousness of the deceased might still be in the body or around nearby, yet to take rebirth. There should be more weightage on chanting of Amituofo’s name, over chanting of Amitabha Sutra. Yet, the sutra is still important as the deceased might not understand the rationale of Nianfo yet, e.g. if he or she did not pay enough attention when alive.
These principles of offering the power of merits (功德力) and guidance (引导力) apply for helping other unseen beings in general too. We must not imagine that all we should do, when encountering unseen beings, some of whom might be disruptive, is to do any general chanting to dedicate merits to them, so that they end their disturbing activities. While merits shared can offer some well-being, it is limited, and they have yet to be guided to a better rebirth. As such, the streamlined practice of chanting Amitabha Sutra followed by Amituofo’s name will always prove relevant.