Expect is useful for automating processes in the background such as through cron. By using Expect with cron, you can not only automate the interactions but also automate starting the process in the first place. For example, you might want to copy over files each night between two networked but otherwise unrelated systems. And you can be arbitrarily selective about choosing files. You might want to copy over only executables that are less than 1 Mb and were created on a Saturday unless the current load exceeds 2.5 and the previous transfer took less than 5 seconds, or some other such complicated set of rules. There are no ftp clients that have such complex front-ends. But you can write Expect scripts to make whatever decisions seem appropriate while driving ftp.
It is even possible to have a cron process interactively contact you while it is in the background. For example, an ftp script may need a password to continue what it is doing. It can search the network for you and then ask you for it. Or you can have it look for a set of users. After getting the password, Expect will go back and use it to complete the task. By having Expect query for passwords, you do not need to embed them in scripts.
Doing backups from cron is another common reason to use Expect. If the backup program needs another tape, an Expect script can tell it to go on (for example, if your tapes are physically longer than the backup program thinks), or it can, again, contact you for assistance.
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