In real pinball, once you've played the game a significant number of times, you'll start to identify certain areas in the game that are trouble spots where you are likely to lose a ball. Usually these are oriented around the two outlanes on the side, particularly whether a given ball is going to end up going down the outlane and not the inlane. Each game has slightly different dividers between the lanes, different bumper angles and constructions in the vicinity of the lanes. But regardless of the specifics, applying a slight nudge at just the right time can help influence the ball into the inlane instead of the outlane. Some games also have certain ramp shots that will drain unless they're nudged on the way out of the ramp, especially for a shot that doesn't have enough force and doesn't clear a ramp.
In some games a little bit of nudge is useful for keeping the ball dancing between jet bumpers. But most players only take the risk of a tilt on heavy nudging when they're trying to save a ball.