The primary reason 8th grade students in the United States often have two monthly exams is to promote a consistent and comprehensive assessment of their academic progress. These exams are designed to provide a structured format for evaluating students' understanding of the curriculum, ensuring that they are on track to meet the rigorous standards set by the state and national educational bodies.

Monthly exams allow educators to monitor student performance over time, providing valuable data on individual progress and overall class performance. This frequency of assessment helps to identify areas where students may be struggling early on, allowing for timely interventions and support. According to a study by the National Center for Education Statistics (NCES), students who are regularly assessed tend to perform better on standardized tests and have a better understanding of the material being taught.

Data from the NCES also indicates that monthly exams can help to reduce the stress associated with end-of-year or semester exams by spreading out the assessment pressure. This approach can lead to improved student engagement and a more positive learning environment. Additionally, the frequency of exams can help to reinforce learning and retention, as students are regularly tested on their knowledge and skills.

Moreover, monthly exams are often aligned with the Common Core State Standards, which are designed to ensure that students are mastering essential skills and knowledge at each grade level. By having two monthly exams, schools can ensure that students are consistently meeting these standards throughout the year.

In conclusion, the two monthly exams in 8th grade serve multiple purposes, including monitoring academic progress, providing timely interventions, reducing stress, and ensuring alignment with state and national educational standards.