One of the difficulties with Expect is that it's hard to explain its usefulness to newcomers. Expect itself is easy enough to understand; but a common initial reaction is, "So?"
Part of the difficulty is that Expect particularly shines in occasional circumstances that no one wants to repeat. It's a tool, more than a product--like a funny little jig that looks peculiar, but saves HOURS when you are doing particular operations with a drill press.
Network devops, for instance, might need to monitor and update hundreds of devices spread across multiple datacenters for a particular task accessed through a command-line interface (CLI). The CLI is "easy", in that it only takes thirty seconds to do one instance of the chore--but a royal nuisance to repeat for hour after hour. A "correct" answer probably involves setting up authentication key pairs and ... well, you can see where this is going. How "correct" is that kind of configuration, though, when it might have to be done only once in the lifetime of the datacenters? This is a typical case where twenty minutes of Expect scripting can save HOURS, and sometimes days, of error-prone typing. It would easily be worth hundreds of dollars to have Expect on hand for this one use. No one will ever pay for Expect, though, because it helps most for these pesky side-tasks that no one really budgets.
I can give lots of examples of good uses for Expect--but each single example will only apply to a handful of people around the world. Nearly everyone needs Expect in some way, but each instance is highly idiosyncratic.
Expect is a little like duct tape or WD-40.