A lot of people will buy books simply because their are for the Realms. Personally I've never liked the Realms much, but they are popular, and it would be silly of wizards to not publish something they know will sell.
These cataclysms do two things. They make the setting a bit more simple and easy to enter, and they help explain(hand waive) the rules changes in the setting. 4e dramatically changed how magic worked, for example, and now it's going back to a vancian model.