Some programs behave differently when run interactively and non-interactively, and they do so intentionally. For example, most programs prompt only when running interactively. Non-interactively, prompting is not needed.
A more serious problem occurs when dealing with programs that change the way they buffer output depending on whether they are running interactively or not. Programs using the standard I/O library provided by UNIX automatically buffer their output when running non-interactively. This causes problems when you need to see the output immediately. Expect can make the programs think they are running interactively, thereby resolving the buffering problem.
As another example, shells force the user to press control characters (^Z, ^C) and keywords (fg, bg) to switch jobs. These cannot be used from shell scripts. Similarly, the shell running non-interactively does not deal with history and other features designed solely for interactive use. This presents a similar problem as with passwd earlier. Namely, it is impossible to construct shell scripts which test certain shell behavior. The result is that these aspects of the shell will inevitably not be rigorously tested. Using Expect, it is possible to drive the shell using its interactive job control features. A spawned shell thinks it is running interactively and handles job control as usual.
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