[beat]

First, not everything produces bitwise identical results from build to build (Websphere ear files, for example). Second, it's time consuming to rebuild from scratch every time. This is especially important if you're in a cloud environment and scaling horizontally for load. You want a way to bring resources online quickly.

[icebraining]

All clouds I've used have snapshots - it's hard to get faster than creating a VM from one. You can just keep an up-to-date template using any config management tool.