Consistent configuration across your environments (for example, dev, test, and prod) means that you can more confidently deploy your infrastructure. When you know what configuration is in use, you can easily test changes in other environments due to a common baseline.
IaC is not the same as just writing scripts for your infrastructure. Most tools and services will leverage higher-order languages and DSLs to allow you to focus on your higher-level requirements. It enables you to use advanced software development techniques, such as static analysis, automated testing, and optimization.