@bwenyenye thank you for the compliment
Now about your question, "Why do we say that may peace and blessings of Allah be on him"?
First is because God (Allah) says in the Quran that we have to say. So we say it because it's God's commandment.
Secondly however, there's a logical reason. The reason Allah is asking us to praise is because it is normal human tendancy that the person who is very famous, who has been known for a particular activity, we tend to follow his guidance.
So we ask the prayers on Allah's messengers and prophets because Allah sent them to us.For example, supposed your mother has heart attack (God forbids) and if you know that there's heart specialist who's the best in the world.
So when you want to go and take treatment for your mother and ask how she will be cured, will you ask the cardialogist, or will you ask an unknown person or will you ask a quack, assuming all are free. Who will you go to, the best in the world or to a normal doctor or to a layman? Of course the best cardiologist because you know it's the best.
Similarly we send blessing so that we ask Allah to bless him for his good works.Whenever we say may peace and blessings of Allah be on him, we are agreeing that he was one of the best human beings in the world.
Even the other messengers and prophets were the best too, we say may Allah be pleased with them as well.
That means these messengers and prophets, may Allah be pleased with them because we know that these people are good people.
The moment we acknowledge that these are good people, so whatever they say, we have to take as guidance.
Using above analogy, when you go to heart doctor, and he gives you treatment, you follow the treatment.