In the 1950s, the military adopted a single exam known as the Armed Forces Qualification Test (AFQT). Used as a screening tool, the AFQT measured a recruit’s ability to absorb military training and their future potential. It was supplemented by service-specific battery tests for the purposes of MOS classification. In 1972, the Department of Defense determined that all services should use one exam for screening and assigning individuals to an MOS. The AFQT was phased out over a two-year period in favor of the current Armed Services Vocational Aptitude Battery (ASVAB) test.
A good score on the ASVAB is different than a minimum required score.  Each of the military branches will have their own minimum required scores (see below).  In practice, however, each branch will be more selective in their recruiting.  A score of 50 on the ASVAB implies that you scored as well or better than 50% of comparable test-takers.  Since ASVAB scores are used for many purposes (e.g., enlistment eligibility, military job placements, and career exploration), it is important that you score well on the ASVAB.  A score of 60 or better should be your minimum target.
The ASVAB is one of the most widely used aptitude tests in the world. The intent of the ASVAB test battery is to assess a candidate's potential for future success in the U.S. Military. Because of the nature of the test, the ASVAB can also be used to give a candidate valuable information about both military and civilian career choices that they may be suited for.
If you're looking for more information on the ASVAB, we've also written extensive articles about the test to help you feel confident on test day. Read through these informational ASVAB resources at any time to learn more about the purpose of the ASVAB, important concepts related to ASVAB qualification requirements, passing scores, registration process, test-taking strategies and much more.
After adding so much information to the aptitude test, there was a bit of difficulty interpreting the test results. In addition to that, a vast majority of test takers were deemed as being under qualified based on their test results. This is why the percentile change was made. It ensured that a 50% actually correlated with a person doing better than 50% of the test takers. Those revisions have worked hand-in-hand with the preparation of the armed forces.