Ever get a letter or a card from a loved one with the date written on it, like "6 September 2008"? Unless you get electronic mail that automatically dates correspondences for you, and/or unless you are blessed to have a friend or loved one for whom the date something is penned is important, you are not likely to know when their letter or card is written. And years later when you pull out the old letter, when the date is there, it will assist you in remembering what was going on around you at the time. Chances are, older friends & relatives will always date a letter.
I've been richly blessed by three paternal aunts who dated every card or letter I ever received from them. I suspect since they grew up in a era when personal correspondence and diaries were commonplace, they dated things with the same regularity you and I sign our credit card receipts, or our checks, if we still write those. I appreciate the strange comfort of reviewing these old letters and notes, wherein some actually gave the day of the week. That thoughtfulness was modeled well for me; therefore, I strive to do the same and pass it on to the next generation.