Automated testing IS a development tool
Alot of people share the opinion that automated UAT and unit testing are merely quality assurance strategy that is replaced by manual effort (in most cases; spent by the client/stakeholder).
It's paramount to understand that automated UAT and unit testing are in fact an integral part of the development process, and indeed a development tool. Robert Martin said it perfectly that no one will ever write clean working code on first attempt. Automated UAT and unit test are the things that allow developers to confidently clean up, fix up and extend existing code.