English (language): Why do we use the past tense to show our politeness?

To give a pragmatics answer to why you would use either a conditional model or a present model in questions, to begin with:

In many cultures, and English is one, indirect requests are considered more polite than direct requests. An indirect request implies a direct request, but it gives the listener the (fictional) option of backing out by misunderstanding it, and it also gives the listener the (conventional) option of offering to fulfil the request, rather than being seen as complying with a demand.

“Pass the salt”. (Oh, that’s a direct request. Can’t get out of that. I have been put on the spot. Damn blunt furrner. Should get Trump to slap an armband on him.)

“Can you pass the salt?” (I can. Why would he ask? Might  he need salt? Why, in that case, let me spontaneously and completely off my own initiative offer the salt! I feel so  empowered!)

“Could you pass the salt?” (I could, hypothetically, if the need arose. Why would he ask? etc etc…. Oh, and he might not have needed the salt after all, since he was being so hypothetical about it; so kudos to me for being so proactive and able to anticipate his needs!)

Cultures do funny things with requests, since they can be seen as confronting and invoking a power differential. Greek, by contrast, usually limits the indirectness to a question (“Will you pass the salt?”), but it uses other means of toning down the threat to the listener’s face. Like sticking a diminutive on the object requested. Μου δίνετε το ονοματάκι σας; “Will you give me your eensy-teensy name?” There’s nothing eensy-teensy about most Greek names; but that creates the fiction that the request for the name is hardly any imposition at all: I’m only asking for this one tiny thing…