When people die, they leave behind more than just "things". More than just photos and furniture and money and houses and pets and clothes and clutter. They leave behind things that were never said (and now can never be said), they leave unsettled grievances, misunderstandings and the results of actions or inactions that are left to be interpreted by those who are left behind.
When you are thinking about putting your "affairs in order", think about doing more than just signing a Will, a Trust, a Power of Attorney, a Health Care Proxy - those documents carefully drafted by your attorney. Think about writing some of your own documents and keeping those in a safe place to be opened only upon your death (and changed by you whenever you need.)
If your children are young, you could write about what your values are that you hope to pass on to them, what your dreams are for them. If they are older, you could write about why you have structured trust distributions to be made in a certain way (such as for college or a down payment on house, but not a trip to France). And if you have made vastly uneven bequests, or left someone out entirely, I encourage you to write about why you have done that, as well. Often times the parent will say "She'll know why I'm doing this, she'll understand." But I can pretty much guarantee you that a child (no matter what age) who has just lost a parent and then found out that he or she has been partially or totally disinherited will not understand.
And since you are not around anymore to explain, their grief and anger and shock may cause them to lash out at their siblings, their other parent, even their parent's advisers looking for answers. And you are the only one who could have provided this insight.
In writing the letter, you may find that you go through many drafts. You may find that your reasons become clearer and stronger. Or you may find that as you write they seem different, smaller, less important than they did before. Maybe the writing of the letter even causes you to change your mind. Or maybe it doesn't, which is fine, too. But at least it will provide your loved ones with some insight and guidance, which only you can provide.