[msclrhd]

If you are implementing a specification (e.g. CSS/HTML/other language parser), tests are invaluable to assess how conformant your code is. They allow you to test the different parts and specifications in isolation (the lexer, the parser, the value/unit handling, etc.)

If you are implementing interfaces that plug into some other program (e.g. an IDE, or even different teams in a project), tests are invaluable to check that your code works as expected, and does not break when a new version of the program is released.

Yes, writing tests takes longer. Yes, writing tests makes it harder to change the code in the future. However, they have helped me identify cases in my code that I had not considered (when integrating my code with an IDE), and to prevent issues when refactoring and extending the code.

There are ways to mitigate the issues of changing the code, such as creating scaffolding to bridge the old and new code via adaptors, etc.