A common situation is when you have children from a previous marriage. Trust often becomes irrevocable on the death of the first partner and cares for surviving partner but preserves assets for children of the previous marriage.
AB by-pass trust can also be used to double the federal estate tax exemption. Now that the number is up to $12MM, not a concern for many. But way back the number was $600K. And there is discussion of reducing the current exemption or notes that it expires and returns to a lower number in a few years.