You want something more in-depth? Fine.
Please don't delete things. It defeats the purpose of a journal when parts of it start disappearing. A journal is a record of events, thoughts, and feelings, and nothing more. If you make it out to be something else, then you're just fooling yourself. It may server another purpose, but it is still just a record when you get down to it. When you delete or modify old entries, you'r ejust re-writing history. If it's not what you wrote at the time, it's no longer an accurate portrayal of that point of time.
Thinking through all the reasons to deface a journal, I can't think of a single valid one. For example, some people are ashamed of the way they once were. Why be ashamed of what you were, if you are proud of what you are now? Erasing the path you took doesn't make your current status any different. Instead of being ashamed of your history, be proud that you have overcome that history to get where you are now.
Another excuse I hear is a person wanting to forget a certain series of painful events. I would suggest to such people it is best to keep thos events on file, but don't look at them for a while. After the dust has settled, it's often rather therapeutic to go over what you went through some time ago.
Any other reasons to destroy history out there? I bet none of them are really very valid.