What, if anything, is accomplished by formal assessment?
People who promote quantitative education assessments believe that "educational achievement" is a valid research construct  (see: https://en.wikipedia.org/wiki/Educational_assessment)  With the proper instruments (a.k.a. tests and scoring systems), and with proper analysis (norms, scales, etc.), they believe it is possible and useful to draw inferences about the education system as a whole, or in part.

Broadly speaking, the advocates of standardized testing strongly believe that the only way to improve the education as a whole is to have "objective" measures of "reality" across teachers, across departments, across schools, across geographies. This "ground truth" data provides the basis for deciding what works and what does not, and for determining who is effective as educators and who is not.