In the Catholic church, the Mass for the dead was previously know as a Requiem Mass, and all of the rites surrounding that were referred to as a "Requiem". Exequy
Do you mean 'why are last rights important to Caholics?'If so, I believe that it is important to them to allow the person who is dying or recently dead to have the chance to ask God for forgiveness...
Although some people may have personal superstitions about snow at a funeral, the Catholic Church does not officially teach that this has any special significance.