This question already has an answer here:
I am wondering, is there a way that enables me to customize how my friends in my friend list see each other on Facebook?
There are some cases that make this feature so useful and necessary, for example:
You add a new girl as a friend and don't want for her to see your other girl friends in your friends’ list.
You add your current manger and don't want for her/him to your ex manager in your friends’ list.