For the most part, I use it when I'm doing large-scale migrations where the customer has multiple terabytes of data and not enough network bandwidth to send those files. I'll load those onto Snowball, send them to Snowball, and then sync once it's there. Sometimes the truck's faster than the Internet. I've used it in other scenarios.
I have a customer that also uses it regularly. They have what's known as an edge location. It collects large amounts of data over four to six week periods. They rotate pulling one Snowball out and putting another in, then send the Snowball off to get the data into their systems. There are other scenarios where I've used it.