Question: What is an ancestral tablet for? Is it a must?
Answer: Within 49 days after death, it is useful as a focal point, on the shrine of the deceased at home, for making food offerings, offering guidance and support-chanting. A traditional tablet is not essential. A photo will do too, or a piece of paper (or card), with the name of the deceased. It need not be fanciful.
After 49 days, if there are no signs of yet to be reborn, the tablet is optional. It can be retained as a reminder to keep creating merits for the deceased, to alleviate suffering and expedite a better rebirth, in case rebirth is in a less fortunate realm. A tablet at a temple is an optional bonus, as it is not very likely that the deceased will be there, unless there is particular attachment.
When the consciousness is not near, merits can be shared, but there is no guidance received, even if offered. For reaching Pure Land, there should be BOTH guidance and support-chanting offered, through this recommended sequence: https://purelanders.com/2016/07/19/what-should-be-chanted-during-after-a-funeral
See Section : (Urn &) Tablet