People expect effort to lead to performance to lead to rewards.
Expectancy theory is about fairness. In work, we expect if we put in the effort, eventually that will lead to better performance. We also expect that if we perform better, we should be rewarded for better work. When managing teams, it is important to recognize great performances. You can do this through recognition, awards, gifts or even financial incentives.
In game design, expectancy theory explains why gamers think games are unfair: If they put in the effort to learn the controls and systems, and execute correctly, they expect results, i.e. progress. Players should be rewarded for performing well in game: There are a variety of tools devs can use to accomplish this like progression mechanics, achievements, and in-game rewards. Unfortunately, bugs and poor design decisions can break this flow. It is important to both minimize bugs and create consistent gameplay where expectancy theory is fulfilled.