Many people do keep diaries for the purpose of reading them later and remembering/keeping a record of past events. They can be valuable for historians, especially if the diarist is unusual for their place/time in keeping a diary. Martha Ballard is kind of a canonical example of this -- she was an 18th-century midwife in Maine who kept a diary, which would form the basis for this book. Now, Ballard was certainly unusual in being a literate woman during her time period who also kept a diary, so there's a great deal of argument over how representative her particular experiences were -- the book was something we read in my historiography class in grad school as it stimulated discussion of those questions.

Moving to more typical diarists, the example from my field that comes to mind is Samuel Pepys, who kept a diary for 1660-1669. Pepys' diary is extensive and wanders quite a bit, but there are certainly indications in it that he realized that he was writing for posterity. He was involved in naval administration, and also lived through a tumultuous time in English history, and there are definitely descriptions of large events (the fire of London, the Great Plague) that seem written with an eye toward posterity; but he also records mundane events like the time he woke up in the morning and personal events like his pursuit (today we'd call it sexual assault) of servants. There diary has been published in multiple editions, including one that has portions of it you can read online.