salawaat

Meaning of Salawaat upon Nabi (sallallahu alaihi wasallam)

Q: My Question is that why we say 'Peace Be Upon Him' for Holy Prophet? Is Holy Prophet not in peace and safe?

In the Holy Quran Allah says that when beleivers come to you say to them ''salam alaikum - may you be safe" so this verse is for living people not for those who have passed away and we are sure that Holy Prophet is safe and in peace. So why "Sallallahu Alaihi Wasallam"? As generaly given refrence is that Allah says in Holy Quran that I read and My angels read salat to Holy Prophet so you should also read but actually the sense and meaning of this verse is "I and My angles help Holy Prophet and you should also help him" Please explain.