If you do not have children -- maybe you just don't have them yet, or maybe you don't plan to have them at all -- it is still important to write a will and do your estate planning. However, this becomes especially important if you do have kids. It raises the stakes.
Prior to the birth of your children, the will is more about you and your assets than anything else. It's just a way to make sure that your assets go to the people you choose, to reduce the tax debt of your estate and things of this nature. It's almost business-like in nature.