Well, I can't say that I know the reasons well but I'll try to answer your question.
1. Muslim fast because it is stated in one of the commandment of Islam. I think its the 3rd Commandment. Fasting in the month of Ramadhan. Practice of fasting is not recent since it has been practiced by the previous prophets such as Moses and Abraham before Mohammad.
2. Pillar of Islam is just an expression ( I guess ) because in the month of Ramadhan, we Muslims considered it as a holy month. Performing prayers five times per day is also the Pillar of Islam. Pillar of Islam may bring the meaning of strength in the religion.
3. I don't quite understand what are you trying to say there. Do you mean why Muslims' fast are without drinks and foods until sunset? Fasting during the day while not having a drink or eating is to show faith toward Allah.
4. Well, fasting can generate piousness. Its a logical situation, fasting is not to eat or drink for a certain period of time. How you'd do that? By controlling your desire, when you can control your desire to eat/drink that means you can control your other desire. Its like a training period for us Muslims to always remember Allah and control our desire for the earth.
5. 1st, you need to know that there are 2 types of fast
1. Wajib = Compulsory
2. Sunat = Recommended
In Ramadhan, we Muslim are wajib to fast. Why is that? We need to take a look at the Islamic Calendar. Ramadhan is the 9th month in the Islamic Calendar, and it is the 3rd last month. After a year of eating and drinking, it is good to rest your digestive system for a month. That is from the scientific fact.
That is all I can answer. All of my answer is based on what has been taught to me. You can also refer to books that have a complete explanation.