Adding a to_s method to a Ruby class helps with debugging.
I more and more tend to add to a Ruby class I want to test is a to_s method returning some sort of information about the underlying object.
So whenever I have to dig a little deeper and have to debug code that uses an instance of that class or I simply want to write some details to the logfile, I can use the returned string and thereby make sure all relevant information is printed.
Returning only a hard-coded string or name doesn't make any sense at all though. So, let's assume we have some sort of business object, for example a job doing some kind of work in the background. It has a certain state which we want to know about:
If you put this into an application which keeps the waiting user up-to-date with the current state of processing (as simulated below with a Ruby irb console), the user knows exactly what's going on.
There are more methods a Ruby class should have. Make sure to take a look at Robert Klemme's "The Complete Class" article. I don't think a class should contain all of the methods described there, but it's a good starting point.