For making the Queen laugh.
There’s a reason they were married for 75 years. Elizabeth was first attracted to the handsome young military cadet but over the years, despite the often tumultuous relationship they had, Philip’s real role in the household was to keep the Queen in good spirits. It’s like those skits on SNL when it’s obvious that one of the players is trying to get someone else to break character. If you see the Queen smiling, it’s almost always because of something Philip said.
To most of her family of the same age, she was “Lilibet”, the way she said her own name before she mastered its pronunciation. To Philip, she was just “Bet”, and he was the only one who called her that.