For me it is idea of her letting her inner slut out, and having that charge of sexual energy it brings to our own relationship.
I'm the dom in our relationship, and the one in control of who she fucks and how often. Allowing her to fuck someone else is a "treat" or "reward", given to her for being obedient and serving me well as my sub. It fits well with our D/S relationship. She likes to be spanked and will do naughty things without my permission just to get spanked.
Last night she got on the bed, bare ass up and told me "I was naughty today". She said I was on craiglist and sent emails and pictures of my pussy to some guys with big dicks. One guy wants to tear my pussy up with his 10" dick. Feel how wet my pussy is. With every word she said, her ass turned a bit more red from the spanking making her even more turned on. We had a great sex session for several hours after that.
Her having sex with others is very small part of our sex life ( as is swinging ). She has asked to be shared more often, but I limit it enough to keep the sex hot between us and strictly as a reward. We view this is just an activity to enhance our own sex life.