Work experience is not really required to apply for a PhD. Actually, sometimes it might be held against you.
PhD programs are geared towards a career in academia, not industry. So, if you have a career outside academia, but want to change that to become a researcher (thus a PhD), you will have to stop and think about it. And find convincing answers to questions like "why do you want to change?" and "how your experience may help in your research?".
In my case, I applied to a PhD after an executive career of nearly two decades in the corporate world. I am really done with that, I want something else for my life. However, my research interests are related to problems I was not able to solve during that period. That helps to explain why I'm interested in research, and why such subject. Understanding about real world problems may also be important to conduct relevant research.
Work experience may be much more valuable for some types of PhD, schools, and professors. If your PhD is much more related to theoretical research, professional experience may be a hindrance, because you probably come with strong professional biases. If, on the other hand, you will conduct applied research, working in partnership with actual companies, to solve their real world problems, that experience may come in handy.