My experience is that whether one is writing tests first or not, the code that is being written does get tested one way or the other.

In other words, people (for example web-devs) who don't write tests first tend to "test" their code by clicking around in the browser to see whether they get the desired effect.

Writing tests, running them and then iteratively updating/fixing the code is IMO so much more efficient than manually clicking around to test one's code.