It can be a very frustrating part of the game.
If you are unfamiliar with what a sitter is when playing online you have the opportunity to sit out a hand to use the facilities or get a drink. A sitter is someone who has no intention of playing or will only join the table when the stakes get high and they are at risk of being blinded out.
It seems very odd that someone would pay money to get into a game without intending to play but it happens. To alleviate some of the frustration keep in mind that not all sitters are deliberate sitters. Imagine signing into a game and just when it comes up on your screen one of the kids falls down the stairs and needs to be rushed to the hospital. Another nondeliberate sitter is the person who is all ready to play only to have their internet drop out. Sometimes people are forced to sit out by people coming to visit or a phone call from the boss. The point is not all sitters are deliberate sitters.
Don't get me wrong there are deliberate sitters. Usually there are three reasons why a person would sit out during a game.