When most people think about the assets in a trust, they consider monetary assets. Trusts can be used to pass money down to heirs while still retaining some control over that money. For instance, the trustee who administers the account could be instructed to pay out a specific annual amount over a course of 10 or 20 years, rather than just leaving the lump sum to the heir all at once.
Since an estate plan largely focuses on financial details, many people assume that it should be private. Just as it is not considered polite to ask someone how much they make in a year, family members do not want to pry into someone else's personal business. An elderly person may see no reason to tell heirs exactly what the estate will leave to them.