Because it gives peace of mind, both to you and your loved ones to know what you would want in the event of your death.
Families we see at the time of a death can agonise over what is the best thing to do, if not discused beforehand.
For example the simple question we start with is by asking, 'Is it to be for Cremation or Burial'
Unless this first point is settled we can't proceed any further as the answer dictates the questions for the rest of the funeral arrangements.
So by taking the first step and making your wishes known, while perhaps you are still in good health will help everyone.
People do not think as clearly in a time of bereavement, the shock and emotion gets in the way of any clear thinking
'Do the thinking now so they don't have to'