One reason for doing it is that download servers will be absolutely slammed on release. A pre-order will often times come with the bulk of the download happenin

One reason for doing it is that download servers will be absolutely slammed on release. A pre-order will often times come with the bulk of the download happening a few days in advance, and then a very small download at release that contains a small amount of data that makes the game playable. This is an argument for buying a week out, not 5 months out though. Another is that there are very often below-RRP deals for games in advance, but if you buy it on launch day you'll be paying RRP.