There are as many answers to this as there are swingers.
People swing because:
It is fun.
It satisfies a natural human urge to have multiple sexual partners.
To try out fantasies a partner is not interested in.
Because it puts a spark back into the relationship with a life partner.
Because a loved life partner is unable to have sex due to long term illness or injury.
To satisfy a fantasy.
Because an open relationship is less stressful and less damaging than a cheating one.
Because you and your partner have discussed and like the idea.
You should never swing because:
Your relationship is failing - swinging will accelerate the end, not resolve your issues.
Your partner told you to - In swinging you always have the right to say NO. Swingers only have a sexual interaction with someone you they attracted to.