It also indicates that someone is able to follow the rules, and go to school/work everyday regardless of whether they feel like it. Since attendance is legally mandatory, many employers wonder if the individual has a problem with authority if they don't attend school. Getting a diploma also indicates that social skills are at least somewhat reasonable, as they were able to get along with their teachers and classmates well enough to attend for 12 1/2 years.
Basically, the diploma is used by employers to filter out employees who may not be willing to follow the employers instructions day after day.
Some jobs do require math skills and such. Employers will often give math and vocabulary tests to prospective employees to make sure they actually paid attention in school. As well as a physical fitness test, and drug screening.
Their goal is to find someone reliable, compliant, able to learn job skills, and get along reasonably with their co-workers/customers.