Sam Harris and Mark Oppenheimer are sparring over the question at The Economist. Here's Mark:
[R]eligion responds to a deep, satisfying human need for ritual. Throughout human history (and certainly among my three young daughters, who are the nearest evidence at hand), people have liked occasion, routine, ceremony. We like regular, predictable occasions to come together, offer thanksgiving, celebrate common history and experience, and affirm our ties of community.
Such rituals do not have to be religious, of course: there are civic rituals, which in America include Independence Day (and its fireworks), Thanksgiving (and its meal) and Memorial Day (often with a picnic or barbecue). But many of the best, most enduring rituals are religious: Christmas, Easter, Sukkot, Passover, Iftar, etc. And it is worth noting that even supposedly secular rituals tend to accrue quasi-religious elements to lend them meaning: prayers, invocations, discussions of a people's "destiny". In other words, it is hard to keep such rituals purely secular, although I am sure it can be done.
By the way, the best religious ritual of all is the Sabbath, and it so happens that religious people are much better at keeping a day of rest than secular people who make periodic resolutions to keep a "secular Sabbath" or just to "slow down". It seems to be a particularly, if not uniquely, religious good.