It can be used before or after regardless of its strength.Maybe some women want to take creampies (for a lack of less vulgar word)? Maybe they like that feel without being made pregnant? Science argues women who do take creampies (there's that word again) can be healthier and happier as a result.
It's not an abortion pill. It prevents the ovaries from releasing an egg and makes in harder for the sperm to get to one. It's preventative and only works if you're NOT already pregnant, hence having to use it so soon after sex.