I finally found out what these are:
I asked Naomi. She said they can be notices to tell you that someone has died or for basically anything else. You rent them (around NT$500 for one) to display a notice. So if a friend or family member has achieved something, like getting elected into the local council, or opening a business, or anything, you can rent one or however many you like and display a notice.
Pretty cool to know :)