Probably she feels that some of them are worth keeping and reminds her of some sweet memories. Who knows some of them might be from her ex, friends or even from you. I keep some messages for years, just a simple "happy birthday" from my brother or some encouraging words from friends. I look at them whenever I'm down and they comfort me, it's a little secret to me. I do not have habit of showing my messages to anyone, not for any other reasons, the message is meant for me, I read it and is how and what I feel. The same message might mean a different thing to different people, I do not see the need to show it to a third person and make plenty of explanations. I feel that it is also a kind of respect for the sender, how would you feel if you send a message to your friend and is purely only for that friend, not to someone else but the third person know the content of it.