I am now 80 and have kept diaries since I was about 9 years old. Some have very little that would be of interest to anyone else, but reading just an odd sentence I can be transported to that particular time. I have quite a good memory and not just what was happening , but often sense of smell of something or the lovely little flower buttons on my sisters blue dress etc etc.
When I lived abroad and in particular in two y ears living in Portugal I wrote to my family the whole time. Not a set amount each week, but when I had the time, and once after a particularly busy time , when I was singing in concerts and making classic recordings etc etc wrote 24 foolscap pages home. I used to put all the things that we were doing and also what was happening in things like fiestas etc etc and a careful comment or two on the political situation (Salazar was the dictator then) My mother kept them all and then eventually they were passed to a library with the information as to what I was doing there etc and now it is not the information about what we were doing, but rather the information and knowledge about the life in the country , at that time, that is of interest. I also lived in Damascus for a while, and travelled a great deal, So it takes very little to connect with all these years, and what was happening. You cannot get them back once they have gone, and whilst the internet has its uses, no one knows what may happen in the future. We have seen information lost through malicious intention and also by mistakes. Written information, so long as it is not tampered with and left in its old form is there for many reasons. People in your family may be glad of the information when trying to trace family, or be pleased to see that you also enjoyed a particualr hobby or interest. To me they are truly valuable and I would not destroy them at all. Birthday and christmas cards are not so important, but things like the calendars , I dont know since covid , but we used to give good pictures from a calendar to a play group. They can be cut up into large pieces and show the children how a jigsaw works, or they can cut them out and stick them in a scrap book.I always found they enjoyed having the pictures to do as they liked with them.
For long time information on an earlier time , if someone has lived there can give historians lots of little clues, and I also get people to write on the back of photos what the photo is and also when it was taken. Good job we are not all the same and I think the fact that I can pick a diary out and read a couple of sentences and be taken right back to that time is probably why I keep them.