CNAs perform many tasks while caring for their patients. For the CNA exam, you will be asked to perform any number of those skills (usually about five or six) in front of an instructor. The student cannot know beforehand which skills she will be asked to perform, so she needs to be able to perform them all proficiently.
Bathing is simple enough, but, remembering the incidentals while doing so -- ensuring dignity and privacy, providing for infection control, and effectively communicating -- takes practice. Safe ambulation, repositioning, and transfer techniques, to name a few skills, require rehearsals. Remember to always speak calmly to the client, explaining what you are going to do, before beginning care.
Professionally performing all CNA skills is not only a requirement to pass the test. Your patients will ultimately rely on you to properly and completely perform your duties. Practicing these acquired abilities prior to taking the exam will give you the confidence you need to pass the test.