Aaron Patterson calls himself a “puts debuggerer”. His article lists useful techniques for debugging ruby code using puts:

• I know where I am but not how I got here: puts caller will print the call stack.
• I’m calling a method, but I don’t know where it goes: p method(:render).source_location will show you where the method render is defined.
• I’m calling super but I don’t know where that goes: p method(:render).super_method.source_location will show you where method render’s super is defined.
• An object is being mutated, but I don’t know where: #freeze the object to trigger an exception (with backtrace) when it gets mutated.
• I have a deadlock, but I don’t know where: Inside the thread call p t; puts t.backtrace to get insight.

And then there is always the byebug gem with next, step, etc. when puts is not powerful enough.