Why spend time writing tests? 

Testing programs manually is natural; writing automated tests is not.

Programmers use various techniques when learning to code or when dabbling in new technologies and libraries. It is common to write short snippets, follow a tutorial, play in the REPL, or even use Jupyter (http://jupyter.org/). Often, this involves manually verifying the results of what is being studied by using print statements or plotting graphics. This is easy, natural, and a perfectly valid way of learning new things.

This pattern, however, should not be carried over to professional software development. Professional software is not simple; on the contrary, it is usually very complex. Depending on how well designed a system is, parts can be intertwined in strange ways, with the addition of new functionality potentially breaking another, apparently unrelated, part of the system. Fixing a bug might cause another bug to spring up somewhere else.

How can you make sure that a new functionality...