When looking to buy the latest games, pre-ordering or participating in betas for early access is common practice. Apps have become a large market that is highly dependent on name recognition but without a way to officially accept orders in advance. Apple has decided to change that by allowing developers to put their apps up for pre-order so long as the app will launch within 90 days.
As an end user, buying apps via pre-order seems to make little sense because there is no guarantee that an app will turn out the way you thought. However, there will undoubtedly be iOS users queuing up to buy the latest trending apps, much to the delight of developers.
One of the key benefits to developers is the ability to gauge app pricing during the pre-order phase. Developers are able to change the price of their app at anytime before it is made available to better maximize their profit. Apple does protect users a little in this sense because pre-order customers will never be charged more than what they paid and will receive a discount if the price drops.
In-app purchasing can be setup during the pre-order time period so that all in-app content is available immediately at launch. Developers may begin to offer exclusive content to those that pre-order their apps. In order to force developers to stick to the 90-day span for releasing apps, a completed version of the app must pass Apple's review before it can be listed for pre-order.