As children we found it to be a good way to get treats and Christmas money... My earliest experience as a toddler had strangers at my door singing wonderfully, then my poor parents gave them money...
Now that is curious, must be a local custom. I don't think carolers here would know what to do if money were handed to them !
Some homes may invite them for a cup of tea of hot cider, but if they accept at each house, they would never finish their rounds,
However, this carol singing would be stationary, at one address.