Importance of unit testing

Question

So I have been wondering about the work that consume more time between writing unit test for an application, and fixing bug of an application which has been made without unit test.

Is that a must to write test for any application or could we spot application where test need to exist?

Answer 1

I guess it depends on how complex the application is, the individual pieces of functionality and how bad it would be if stuff started going wrong. For a small personal project I might not bother but a large application with complex logic for a paying client I'd want to make sure everything worked as intended. If anything changes your unit tests should be able to ensure the basic functionality they were looking for.