Journals and tapes

Now nobody will be surprised by the fact that people literally document every step. It has become commonplace and because of this you really understand how many inappropriate people (judging by the fact that they flaunt). Sometimes it seems that the majority of people lives in some shows where there is a struggle for ratings.
However, in the late nineties, the technology did not allow such publicity to everyone. In other words then it was not so fashionable as it is now. And I wonder why Eric and Dylan so much detail to document all their thoughts and preparations. They did it for publicity or they wanted a way to show that no one else involved in the crime (because it really facilitates the work of the investigators). Why Eric writes in his diary is very personal things, which usually do not tell everyone? Why Dylan did not destroy his diary, which originally was not intended for prying eyes (maybe he believed in the "magical powers")?