Quickcheck tests are especially easy to use in cases where you wrote code that operates in two directions. For example, saving then loading a file should result in the same data, or a round-trip through a conversion process shouldn't modify the data. Alternatively, if there are two paths to do something, you can verify they are identical, for example saving a file and loading it compared to online syncing a file to a server, should result in the same file on the other end.