Question: What is the difference between ‘writing (an ancestral) tablet’ (写[神主]牌位) with the name of the deceased and not doing so?
Answer: Writing a tablet is supposed to express the intention to dedicate practices and merits to the person named. (There can also be tablets written for the living, to express dedication of merits for their well-being, such as recovery and long life.)

It shows something tangible for the deceased’s consciousness (if still around) to see, to know they are cared for, and for the bereaved to see too, to show their concern. The tablet also reminds the bereaved to continue caring.
Dedicated tablets in Buddhist temples is a form of tangible product and ‘receipt’ for the sponsored chanting services too.
Question: If there is no written tablet, but still having practices of repentance (忏悔), mindfulness of Buddha (念佛) and dedication (回向) of meritorious virtues (功德), will this do?
Answer: If sincerely practising and dedicating directly, this will already do. Merely having a tablet at home or in a temple without doing anything else does not keep (creating or) dedicating merits automatically.
Note that in the Buddha’s time, there was no practice of using tablets, but it can be useful, due to the reasons above. A tablet or the equivalent in function need not be fancy. It can simply be a card (for durability) simply with the deceased’s name, or even a framed photograph, as long as it serves to remind. Ordinary cards will do, but there are more formal cards that can be purchased at Buddhist shops too.
As examples, in the first picture above are general tablets without any specific persons’ names, for [1] dedicating to the living, for ‘lengthening life and praying for blessings’ (延生祈福牌位), [2] dedicating to the deceased, for ‘rebirth with transcendental deliverance’ (in Pure Land: 净土) (往生超度牌位), and [3] dedicating to all beings, for ‘universally benefitting the ten directions’ (普利十方牌位). (In Chinese culture, red tablets are reserved for the living.)
Those in the second picture are for filling in the centre blanks with names, for [1] dedicating to the deceased, with wordings meaning, ‘May the Buddha’s power offer transcendental deliverance to ___ Rebirth’s Tablet’ (佛力超荐___往生牌位), and for [2] dedicating to the living, with wordings meaning, ‘May the Buddha’s light focus and illuminate ___ Longevity And Good Fortune’s Tablet’ (佛光注照___长生禄位).
Related Article:
Chanting When Dying, After Death, During And After Wakes
死亡前后与丧礼中后的诵念
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