First and most simply, a tattoo is an excellent souvenir that people can take home and post on Instagram. By doing this, they will spread the word about the event and the artist on social media.
Then, there is a slightly more wholesome reason to think about. The purpose of party favours is to give you a physical reminder of the event you attended and enjoyed. The best party favours can make you feel closer to the other people there. So, a tattoo you and your friends got at a party is the most long-lasting and uniting party favour ever. Even better if that tattoo is done by a local artist who gets more exposure and, as a result, more clients from working the event.