Therefore, God has established the ritual ablution on these days as a way to honor these days and to make them superior to other days - days in which many recommendable prayers are said, much servitude is expressed, and it is a way of purification from one Friday to the next.
And the reason behind the major ablution for the dead is to cleanse the corpse from the various filth, illnesses and other things that might have fallen upon him, since the dead person is going to meet the angels and associate with the people of the Hereafter. It is highly desirable for him to be clean and pure now that he is going to meet God and the Pure Ones where they will touch him and he will touch them.
This is so that the Pure Ones may ask the Honorable the Exalted God to forgive him and they may intercede on his behalf.