This test was originally designed as the admissions and job placement test for people who have already signed up for military service. It determines whether a potential recruit is qualified for the military and for certain military jobs.  But now, the Pentagon is offering it free of charge to over 14,000 schools throughout the US.  And many of those schools are making the test mandatory for all students, not just students considering enlisting.  

Why would the government offer a test designed for the military in public schools? To identify pre-qualified leads for military recruiters.   The ASVAB helps to focus recruiters' efforts on eligible students, saving recruiters' time and energy at the schools' and students’ expense.  Recruiters get the students’ personal contact info along with test scores. 

The Department of Defense reported that over half of today’s youth between the ages of 16 and 21 are not qualified to serve in the military because they fail to meet the military’s entry standards.   The ASVAB helps to focus recruiters' efforts on eligible students, saving recruiters' time and energy at schools' expense.  

Here’s a piece about the ASVAB test from the US Army Recruiting Pamphlet, a guide for recruiters:

“As a recruiter, one of your most important responsibilities will be arranging the ASVAB in your schools.... ASVAB is designed to provide the field recruiter with a source of leads of high school seniors and juniors qualified for enlistment into the Active Army and Army Reserve.... The ASVAB prequalifies potential applicants academically before more expensive and time-consuming medical and moral qualifications are done.... It identifies the best potential prospects for recruitment that allows recruiters to work smarter. The printout provides the recruiter with concrete and personal information about the student.”