I don't think anyone sets out to have a "one-night stand" except those for whom the physical/psychological need for sexual gratification is so great they seek the experience but don't want any possible hangups of knowing about the other person and their life.

Many sex addicts and those in the throes of certain mood disorders pursue one-night stands specifically for the sexual gratification.

Too, though, there are the Charlie Sheens of the world who just enjoy sex and find other like-minded people or, there are the dishonest that pursue one-night stands but the other person does not know that is all that is possible with that person.

Some people get drunk or otherwise impaired and engage in such behavior and regret later but don't otherwise change their behavior so it is less likely that they will lessen their inhibitions in the future and have a better chance of not ending up having sex.

Married people might want the extra/"different" sex but not the hassle that comes from disturbing the marriage.