A will is meant to make the lives of those you love easier after your life has ended. It lays out your intentions for the inheritance and disposal of your belongings, whether that includes money, property, stamp collections, or sewing machines. Whatever you cannot take with you can be passed on to the living. Whenever circumstances change such that inheritance will be impacted, you should revise your will.
What is a circumstance that demands a change? That depends. Let's look at some examples.