Some people are going to conquer and move on. Others will find joy in playing over and over. Some people only care to watch a movie once - no matter how much they enjoyed it. Others watch the same movie dozens of times. A majority of people like doing and play and watching their favorite things over and over and over. If a pinball machine has some breadth (multiple ways to advance) then the replay value is much higher. So a good combination or breadth and depth is key.

The way we think about modes is that they are a set of objectives that can be done one at a time. However there are other "modes" that are running in concurrently with the currently selected game mode. Breadth and non-linear, sequential modes are powerful because they give you multiple things to work on at the same time and a virtually unlimited number of ways to approach the game. If you beat the game one way, try starting from a different set of objectives or starting point. I think that's what leads to a more replay-able game.

Lastly, I think some modes are just fun to hit over and over. When you're in "the zone" and the adrenaline is flowing it's fun whether you've been there before or not.