This is a recurring question in software development. Responses to this question usually lead developers to an intense debate confronting several points of view. Even though the question seems complex, the only possible answer seems very obvious to me.
Some say that tests result in additional costs to the project because they stretch development time. I see it differently. Believe it or not, every software gets tested during its development. The option "no tests" may exist in theory, but they are always made in practice. The point is to define how you do them.
Let's take the example of a developer who considers being developing an application with no tests. He implements some form fields, adds a button to submit it, and so on. As soon he writes down those lines of code, he opens the browser, fills in the form fields, and submits the data. Then he checks if something that was expected to happen actually happened. If didn't so, he makes the necessary adjustments in the code until it works. Interacting with an application on the browser to check whether it works or not may wrongly lead to the idea of "testless" development. Although they had been done manually on the Browser, he actually did tests. Notice that to implement even a couple lines of code, it's necessary to test them.
The dilemma between testing or not testing an application is false. The real one is if we should do it manually or automatically.
By choosing to test it manually, you opt for the strategy in which a developer opens a browser to ensure not the new functionality only but also to ensure all the existing ones. This strategy sounds a bit expensive to me. A variation of this strategy would be doing manual tests for the new functionality only. In this last case, it's up to the customers to check if the rest of the application keeps working as expected. It seems a strategy even more expensive to me...
Another approach is to test every new functionality automatically. Rather than using a browser to ensure the expected behavior, you do it by running a suite of automated tests. The effort needed to write automated tests is very similar to the effort spent on the browser. The big difference is that a manual test must be done by a human being, is repetitive, tedious, and takes too much time. An automated test is written by a developer just once and is run thousand times by a machine in a fraction of time an human being would take to do the same job. Finally, a fair strategy.
But why do many people think that manual tests are cheaper than automated ones? My answer is that another illusion is involved in the process. The one in which every developer is automatically prepared to write automated tests simply because he can write code.
Even though they share some required skills, developing an application and writing automated tests are very different activities. Programming is just one of the necessary requirements to write automated tests. Years of experience developing applications does not necessarily mean year of experience writing automated tests. Too much experience developing large applications does not mean too much experience developing large applications with high quality. Dozens of skills needed to build an application do not necessarily represent dozens of fundamental skills to write automated tests. So the next time a developer says it will take more time to develop a new functionality because he needs to write automated tests, beware the extra time won't be necessary because of those automated tests. The developer will take more time because he still doesn't have every required skill to write automated tests productively.