from what i heard, it was used as a welcome manner when visitors entered your home. You were then served a dram in the quaich. It was just a way of saying welcome in my house. Thats what the lady said anyway when i asked her about it in a store where they sold them, in Scotland.
"The Quaich originated in the Scottish Highlands centuries ago and became the favourite drinking cup throughout scotland. Traditionally, the Quaich is used when offering a guest "the cup of welcome" and again when offering the farewell or parting drink."