How it really works behind the karaoke booth:

The DJ or KJ is organizing a certain rotation of sing-along. People are being patient and waiting their turn. By tipping the karaoke host you are rewarding them for the extra effort of putting you in sooner rather than later. They will be rearranging playlists, and putting you in the mix and this makes things seem temporarily out of the normal rotation. Indeed, regulars will notice the shift in the rotation. And this could upset those who don’t understand how tipping works in a karaoke bar. Regulars for the most part understand this practice and know that new faces during the evening help keep the show interesting and fun. I will say it again, karaoke nights usually take place in bars and so tipping is just commonplace in this setting. Get used to it. Another strong suggestion is go early so that you get enough time and opportunities to sing.