They were first used in the 1960s in USA when people were reluctant to give guarantees, because they would have to be disclosed in accounts as contingent liabilities. They provide support without parent incurring legal liability which might affect its credit rating, or have adverse tax or foreign exchange implications. The French view has been that they are enforceable, because otherwise, the idea that a commercial instrument would be meaningless, would be unthinkable. Start with the assumption that in commercial transactions, legal relations are intended. The onus of proving the absence of such intention rests with those who assert it - Edwards v Skyways.
Bank says look at the negotiations and the text of the letter.
Greig and Davis says look at the document against the background practices of the parties. There should be no room in the proper flow of commerce where statements made by businessmen after hard bargaining and made to induce another to enter into a transaction would, without any express statement to that effect, reside in a twilight zone or merely honourable engagement. The whole thrust of the modern law is to give effect to transactions. It is for this reason that uncertainty, a concept so much loved by lawyers, has fallen into disfavour as a tool for striking down commercial bargains. It statements are appropriately promissory, courts should enforce them when uttered in the course of business and there is no clear indication that they are not intended to be legally enforceable.