My mom shared a memory and I love it....
She said she remembers her saying "Well, that's food for thought" many times.
She even remembers a pin she wore with that saying on it.
One typically says that type of saying when they hear something new OR maybe she ALSO used it when she heard something she didn't like & she just didn't want to saying anything bad so she said, "Well, that's food for thought"...and was trying to be polite.
Could be like the saying you learn something new everyday..... and I could just see her pondering over a thought.
Whatever she said it for, it's a cute saying....but actually with deeper meaning.