I checked the meaning of Farfetch'd on Bulbapedia and it says it is "カモネギ", likely to be inspired by "鴨が葱を背負って来る", meaning "something surprising, but convenient".
What does "surprising but convenient" mean? Is it like an idiom or phrase that is often used? Can some examples be given how it might be used in daily speeches in a situation or context?
For example, does it mean good or bad? Can it be: I didn't think it'd rain, but it suddenly did and made my car clean; it really is 鴨が葱を背負って来る.
I saw an example of 「あんな高額な商品を買ってくれた上に、家族や友達にまで勧めてくれるなんて、鴨が葱を背負ってくるとはこのことだ」, but I don't quite get what it means.