If you stop playing you'll miss out on something or be left behind and unable to catch up.
Fear Of Missing Out (FOMO) is an anxiety that occurs when you worry about missing something exciting or important. In games, this can happen when there are timed events. If the event gives a special participation reward, then this anxiety can be heightened. If the player misses the event, not only will they miss the excitement of the new experience, but they will also miss the special reward. If the reward provides a benefit to the game, then the player will be behind everyone else and it will be difficult to catch up.
This anxiety can drive people to play the game at times when they wouldn't otherwise. It may even keep them playing a game long past when they derive enjoyment out of it. For example, a player who has lost interest in a game may see the announcement for the next event and think to themselves that maybe this new event will make the game interesting again.
Many online games are under continual development with new content and updates being added all the time, so there is a constant trickle of new things that players are at risk of missing if they take a break from the game. This is a strong motivator to keep playing. Often these types of timed events provide an incentive to spend money on the game in order to fully take advantage of the event. This enriches the game developers at the expense of the players who may be playing for reasons other than enjoyment.