How can QA prevent defects?

Question

Also according to Software Testing By Srinisvasan Desikan, Gopalaswamy Ramesh or ISTQB text books. Quality assurance is e.g. reviewing products, inspections, walkthroughs to see if all standards are being followed. This is preventive activity. I cannot see how this can be preventive?

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For the references:

defect prevention (Quality Assurance)

_{Software Testing By Srinisvasan Desikan, Gopalaswamy Ramesh}

Quality Assurance (QA) tries to go one step further. Instead of concentrating on post- facto defect detection and correction, it focusses on the prevention of defects from the very start.

_{Managing Global Software Projects - Page 110}

QA deals with prevention of defects in the product being developed.

_{Software Testing and Quality Assurance}