Why did the ancient Tianmen hang a flag on the ancestral temple or memorial tablet?

In ancient times, carrying on the family line was a task, hanging flags was a ceremony, representing the completion of the task, symbolizing the continuation of the family, the prosperity of incense and so on. Customs vary from place to place, and what you said is just one of them.

Oh, you mean the customs in Yunnan and Guizhou, or the customs in Guangxi. I remember one of my buddies from Guiyang said there was such a custom that the national flag said something like the pillar of the country, but I can't remember what it was. The region is different. My home is in the northeast, where the tradition of the older generation is to burn incense, add red rope and burn incense.