Even those that require you give a certain amount of "notice" before you quit your current job (two_week notice is standard) often may not provide any formal requirement that you do so by writing a letter of resignation or explain in detail why you're leaving.
Your End Date. The most crucial detail in a resignation letter is your end date. Legally and according to your company's policy, you might be required to give a minimum amount of notice (two weeks, or possibly more, depending on the company and the role). If that's the case, you don't want there to be any confusion about how much notice you give. If it's right there in writing in a letter dated two weeks before your last day, you're saving yourself the hassle.