I have not heard a definitive reason for this. Listing out all the reasons I can think of, apocryphal or not.
- To check for tea on the bottom before you give it to the guest for haiken.
- To check for poison. lol.
Reasons I’ve heard for why there’s foil under the lid:
- Gold changes color in the presence of poison. lol.
- Ivory is hard, and needs something underneath it to 1) keep it cushioned so it doesn’t rattle, and 2) to provide a better airtight seal.
- Gold fits the bill, and is pretty.
I like the second reason.