"Could you please shut the door?". "Please can you pass the salt". What's with the "can", "could", and "may"? It's so annoying!
When saying "please", it turns a command into a request. As such, the command "Pass the salt", becomes "Please pass the salt". There is no requirement to add "could" or "may". Why is this so hard to understand? Simply, take the command, and prepend the word "please".
The only thing i can think of, is that people do not want to impose, so instead of asking for a command, they are trying to evoke an emotion in the other person. So, whilst "Please pass the salt" is a direct command, mollified with "please", "(Please) would you be so kind as to pass the salt" isn't even a command, it's a question. Mayhap, that removes the "harshness" from the command. Or, put differently, it removes the "do me a favor" and makes it "i beg of your highness a charitable act for this poor soul".
Even so, the more proper way to say that would be "if you'd pass the salt i'd be grateful". Eh, so it's lacks the same effect. But, i guess, i don't really care. People should say what they want. If afraid of the demand, add the word "please". After a favor is granted, saying "thank you, i appreciate that" makes it all the better.