For a lot of people, FWB relationships are a great way to scratch a sexual itch without having to commit the time or emotional investment into a full blown relationship. They are also excellent for polyamorous people who are interested in pursuing multiple different types of relationships at one time.

Why is friends with benefits bad?

Because youre friends, youre probably going to hang out outside of the bedroom, and this means that you will inevitably see them flirting with other people. While thats not a problem for some people, it can trigger jealousy in others, even if you logically know youre not in a monogamous relationship.