A mix of these things-
The cultural aspects are important. I enjoy the holidays, the foods, the ceremonies from my culture. I like participating in practices that have been passed down for generations. Holidays are fun and I enjoy my traditions!
I find the religious services meditative. When at services I believe that many of sentiments expressed through prayer- gratitude for being alive, strength with my community, awareness of the beauty of the world around me- are worth expressing even if there is no entity to receive my words. I benefit from the practices directly.
I appreciate the ethics and moral guidelines in Judaism. As you may know, Judaism can be more a religion of laws and moral principles than faith, per se. I find many of the principles in the Jewish interpretation of the Torah to be meaningful to me and are integral to my core values. That said, I would never fit in a Orthodox or “Conservadox” community. My conservative synagogue, or more likely a reconstructionsit one is a much better fit. Reform is also not a good fit, because they shed much of what I enjoy- the Hebrew, the adherence to tradition.
I like my community. I enjoy spending time with folks who have a similar cultural context as I do.
I wanted to teach my kids so they were knowledgeable and could define their own choices from knowledge, not ignorance. My kids are atheists too, and were open about it at religious school. Respectful, but they didn’t hide their beliefs. The Rabbi’s enjoyed them for their intellectual curiosity, love of their heritage and spirited debates. I was an atheist from as long as I can remember and still felt connection to my culture and my religion my whole life.
Many people are surprised how easy a fit atheism and Judaism really is.