Having a magna cum laude degree is most important for jobs in the fields of finance, management consulting and engineering. These jobs are well-paying at the entry level and quite competitive, with more than 100 applicants for each spot. Using grades is an easy way for employers to narrow the field.
High grades reflect a candidate's work ethic, discipline and intelligence. These attributes are necessary to do good work in any field. In fields that require graduate degrees, including law and medicine, having a magna cum laude degree is quite important in gaining admission to top institutions.
Students from the best graduate schools tend to get the best job opportunities. For most graduate schools, admission is based on undergraduate grades and standardized test scores. For all of the jobs mentioned, grades only matter for the first or second job. After that, experience and accomplishments play a much larger role, as well as references from past supervisors and colleagues.
Essentially, a magna cum laude degree signals to employers and graduate schools that the candidate is capable of working hard and competently. Of course, this does not guarantee success in the workplace, as there is little correlation between career success and academic success.
Career success ends up being more about being able to work well with others, forming meaningful relationships and creating value. These characteristics are different from the drivers of academic designations. However, a magna cum laude degree is an asset that can help a candidate get a foot in the door.