Standardized tests are good because they assess how well the children are learning, if the teachers are teaching them correctly, what the teachers need to teach more of, gives parents an idea of how their kids are doing in school, and it gives the school principal an idea of how their teaching are doing.
A properly administered standardized test is an accurate prediction of the capabilities of a child (and said child well into adulthood). While there are shortcomings to testing, specifically inattention to the test and poor testing conditions, there's no better way to measure general aptitude; metrics need to be used and standards have to be set.
Standardized tests are subject to rapid updates and effective improvements. They promote equality among students who have different backgrounds. Despite the controversy they have received since the date of their birth, they withstood the amount of criticism and spread out across the globe. If it were not for SAT or GaoKao, you wouldn't know whether you are fit for higher education. If it were not for TOEFL, you wouldn't know whether you would survive under an academic atmosphere in an English-speaking country. If it were not for medical Licensing Examination, you wouldn't know whether you could prevent unnecessary deaths when you become a physician.