'Look after yourself'
I'm inclined to say it
a) at the end of a phone conversation with an elderly friend or relation
b) to someone I love and am not going to see for a while
c) If someone is going out of where we are (might be my house, or a meeting, or a social gathering) into the dark, or wind or rain or ice, or to drive a long way, or anything else that might be slightly riskier than leaving on a sunny afternoon to arrive home 3 seconds later .
Why?