High Holy Days
The #1 theme of a sermon at a liberal Jewish temple is (I would say) self-improvement. The rabbi is someone who exhorts you to get your priorities straight, to live a better life. We are to change our focus. We care too much about material things and money-making. We ought to concentrate on our families and on what really matters. If any one theme comes up most often, it looks to me like it's the value of the sabbath--how we ought to slow down weekly, look inward, enjoy family and friends, turn off the computer. After that, there's the theme of caring for the world as a whole--we should do more for the poor, for the environment, about injustice.
Why does the rabbi get to exhort us all in this way? Who is he or she to say how we should live? Of course, the official answer is that the rabbi knows what God wants from us. But it's not actually crucial--we can believe that to varying degrees or even not at all. The God business establishes the practice of having wise people exhort others to live better lives, but what really sustains it, I suspect, is that people like being exhorted to live better lives. We (and I include myself here) like the periodic experience of being asked to assess how life is going, reconsider priorities, make resolutions, especially if the reassessment takes place in a beautiful setting, with evocative music, and so on.
At the atheist meetings I've been to a few times (as an invited speaker) there are some of the same elements--music, food, kids being educated, charity--but a noticeable absence of exhortation. Perhaps I haven't visited enough times to make a generalization, but nobody gets up and tells everyone else they're too materialistic, or that they spend too much time at the computer. Nobody tells everyone else to spend more time with friends and family, or reading, or experiencing nature. Maybe some topics in the program implicitly transmit that message. If a speaker talks about an environmental topic, attention gets shifted from shopping to the environment. But there isn't very much of the theme of self-evaluation and becoming better people.
There really couldn't be, because it's not easy for one person to get granted the exhorting role. That can transpire despite a fair amount of godlessness (as there is in a liberal Jewish congregation), but if everyone's godless, and adamantly so, it's hard to suppress the question "Why should you exhort us?" I suppose there could be godless preachers. Nietzsche wrote in a preaching style, literally commanding people to do this and do that. (See the Gay Science for what must be the world's best atheist sermonizing.) But it's hard to imagine a Nietzsche figure preaching at a freethought meeting. Exhortation to live a better life is "for them, not for us" ... I think that's pretty much the feeling.
I have no empirical evidence that it benefits people to be involved in holidays and rituals of self-examination, but I think it does benefit me, and I enjoy it!