In principle, it’s simple: the slot machine should have a feature that encourages you, the player, to keep playing on the machine. The longer you play, the more likely you are to win. So you are thus rewarded for your ‘loyalty’ to that slot.
For example, you have to collect a certain number of symbols to trigger a bonus feature. This is different from most bonus features, because in this case you never collect the symbols in one spin, but always need several spins. So you have to keep playing until you have them all and the bonus fun begins!
