What is the importance of security testing in software development?

Security testing in software development is like putting a seatbelt on a self-driving car: essential for a safe and smooth ride in the digital world!

I would just like to add, I work in an industry where there is a high importance of security as a result of the data we work with. From my experience, what I see is that just doing "random" security testing is of course better than nothing, but it should really come back to a risk approach. What risks / attack vectors are you trying to protect and how are you addressing those. For example, OWASP top 10 is a great starting point to identify the most common attack vectors for applications. Depends on your app, the data you are working with, etc to then determine what you need to consider from a security testing point of view.