I used to keep diaries in my late teens and was once asked to destroy one that got read by someone who shouldnt of been snooping in the first place. They didnt like what they saw even though it didnt pertain to them. I was gutted I had to destroy it. Years of memories burnt, for nothing. I was always bitter about that.
Anyway as to the significance. Some people keep diaries as memories or reminders. Like photographs. Other people collect momentos of relationships or special times, like keeping train or gig tickets, teddy bears, love letters, old foreign money from that special holiday. The problem is that it all holds sentimental value for the person.
I have noticed two kinds of people when it comes to memories of the past (Esp relationships) There are the people who destroy everything at the end of relationships or eras considering it to be "finished" and whats the point in holding on to these things. Then there are the people who keep these trinkets and memories and hold them dear.
Me and my partner are both "collectors" We both have love letters, photos, cards and other memorabilia from past relationships and past experiences like gigs or holidays etc. The only reason I feel so comfortable with him keeping these precious memories is because I personally know why I keep mine.
My partners first love sadly passed away when she was in her early twenties and he has kept many memories and speaks so fondly of her that sometimes I feel slightly bizarre. (Silly thoughts like, can I compete with his first love, especially after what happened, does he compare, do I compare) but I couldnt ever EVER ask him to distroy anything. It would hurt him and I want him to have those memories, I am big enough to know that he loved and fucked and argued and danced and laughed with women before me but now he loves me. So heck why shouldnt he keep his memories.
I keep them because that was my life once, that was my experience at the time. Looking back through these memories I remember the times I had with people that have since faded out of my life, friends and exes. With diaries, I can look back and read something and think "omg I remember that!" and have a giggle at my naivity or the fact I was stressing about something so irrelevant but at the time it was killing me lol. Its like a little snapshot into the past thinking, "if I knew then what I know now"
The problem is that if you do keep diaries for years, this exact situation arises. From someone who keeps diaries its pretty embarrassing to have known someone read about the bullshit I wrote about the ex, who at the time I was angry with. Time moves on, feelings move on and people split for good reasons and when I look back at these writings I cringe, but I keep the diary because well, its my memories you know? good bad or ugly its me.
For me it is more about remembering a time in MY past that another person shared. It is not about keeping something because I want to hang on to those feelings or to that person. Heck, I have stuff from childhood from first boyfriends and I can sit and giggle and remember how cute it was. Things I would of forgotten without these little trinkets. I even have things kept from bad relationships because they remind me of MY past and experiences.
I guess some of us just have this sentimental thing and it means a lot to us but certainly doesnt mean we still in any way care for ex partners. As for finding out some cringeworthy information, he would probably feel embarrassed if he knew you read it. He probably kept it from you because maybe it doesnt show him in his best colours, or maybe he didnt want you to have issue with his reasons or his ex.
There are many reasons and I must admit it stings when you have been with someone a long time and find out things they havent told you accidently and you think "Why? When I am so honest and forthcoming, did he feel the need to not tell me" but people have their reasons and it is usually to avoid confrontations, to avoid coming across in a bad light or to avoid it causing a rift between people.
As Shellyboo said, the reason he kept it is that is kind of the point of a dairy. A lot of work and feelings and memories go into a diary and you read the last page. One of the ones you didnt want to read. I wouldnt throw out a full diary even if I embarrassed myself with a long rant about something personal at the end, too much goes into them to be discarded like that without a thought.