It may be for backing up infrequently accessed data (compliance logs, etc) for example.
Hypothetical: you create a logging service for users to send all their log data to you. You promise 365 days of archives, but 30 days of data accessible at any time. You create a lifecycle rule on your S3 bucket to automatically archive data to Glacier 30 days after creation. On the 31st day, your user decides they want to look at an old log. They click the big Download button. You display a message saying they'll get an email from you when that data is ready to download.
Hypothetical: you create a logging service for users to send all their log data to you. You promise 365 days of archives, but 30 days of data accessible at any time. You create a lifecycle rule on your S3 bucket to automatically archive data to Glacier 30 days after creation. On the 31st day, your user decides they want to look at an old log. They click the big Download button. You display a message saying they'll get an email from you when that data is ready to download.