If you spend hours preparing for the exam, it makes sense that at some point in your preparation you want to learn aspects of the exam in addition to just the knowledge you will need for the exam. For example, the format, how long the process will take, and whether there may be any knowledge you need more info here to take the exam.
Imagine having less time after studying countless minutes for this type of exam and walking to a testing center to take the exam. When you do, you find that your questions are in an unfamiliar form, think of you to answer those questions, and some questions approach something you never even realized was a thing to do with the exam.
For this reason, preparing for ASVAB is important. That's why you should try the ASVAB sample exam. The disaster scenarios mentioned above can be avoided quite easily. It's as easy as taking a mock exam and following directions. In other words, you don't want to prepare for an essay exam only to find that it has many options or vice versa.
Also, you don't want to assume that you have an hour to complete the process, only to find that you have 45 minutes. Or maybe even worse to find out that the less than expected time is unfamiliar with the testing strategy you need to maximize your time.
Ultimately, one of these factors is enough to cripple the test taker's ability to handle this type of exam well. As a result, all of these factors are reflected in your profit or loss and your financial future.