Sometimes you need to take a whole bunch of data and convert it to just one number. You can do that easily with the reduce() method on a Java Stream object.
Sometimes, all you need to do is go "old school" and just concatenate a bunch of strings. You can easily do that with Java Streams and Collectors.join().
Sometimes, it isn't good enough just to collect data. It's often the case that you need to take that collected data and transform it into something useful.