A testamentary trust allows for a trust to come into effect only upon the death of specific persons mentioned in a will. For example, a person may make a testamentary trust to hold residential property so that dependents can live in the house until they are financially independent, or until their death. This is to prevent the property from being sold prematurely. The property can be sold and proceeds given to the beneficiaries when the trust ends.
A testamentary trust can also be used where beneficiaries are too young and incapable of responsibly handling a lump sum benefit. It can also be used to motivate a beneficiary with payments being made upon the condition that the beneficiary achieves certain goals, i.e. obtaining a university degree.
Leaving assets to children will have the effect of impliedly creating a testamentary trust where your trustee will hold the assets on trust until each child attains the age of majority.