Learning different approaches to testing
There are many approaches to testing in Ruby. It's possible to use any of them or potentially all of them successfully on the same project, though most Ruby projects stick to a single approach.
One approach to testing is manual testing. This is where you just run the program or use the library, and make sure the results are as expected. It was common in older software for this to be the only method of testing. Applied vigorously enough, with strict checklists on everything in the system that had to be tested, manual testing can result in high-quality software. There were entire careers based not on writing software, but only manually testing software that was written by others.
While it is possible to be successful purely with manual testing, it is very labor-intensive to manually test code, compared to having a computer automatically test code. Back when programs were much simpler, programming took much longer, and software releases...