It isn't 100% necessary to work as a CNA prior to becoming a nurse.
The things that it teaches you are
1. workings of a hospital
2. Different roles, responsibilites of nurses, techs, dietary
3. Learning how to interact with patients and families
4. How to operate some equipment - monitors, feeding pumps, suction machines, glucose testing. Some will depend on the facility and the specific units. Some CNAs get to do more, some not
5. Vital signs and reporting VS
6. basic charting
What being a CNA does NOT teach you
1. Daily functioning as a nurse. Although you will work along side nurses, your job is focusing on the tasks assigned to you. You may not get to see how discharge teaching occurs, how discharge planning and preparation work, nursing assessments, determining when and why to call the MD, who are the resources. This is the role of nursing school
2. The responsibility of licensure. As a CNA you are an unlicensed personell. You have tasks delegated to you. You do not learn why some tasks can be delegated and others not. This is the role of nursing school
There are liklely others that can be added to the list, but this is a start.
I worked as a CNA while in nursing school and the hospital I worked at hired me as a new grad because I had good attendance, had demonstrated skills, readiness to learn, and was considered a good team player. I had a good idea of the functioning of the hospital from a CNA perspective, but learning the nursing perspective I learned once I became a nurse.
It is definitely worth it if you can do it.