I'm not really sure if the above poster was just trying to be funny, or what... I don't think it was a very good answer either way, as it was neither funny nor factual.
In a nutshell, Microsoft has only provided a single method for people to boot a machine on an inexpensive non-PXE network and load an operating system from a server. This way was DOS, using old hokey floppies or bootable CD's. Dos is slow, it has a massive list of limitations, and a lot of recent tools just don't work on it any more. But since this was essentially the only way to get a machine on the network (again, in a non-PXE environment) it means that Microsoft had to support it, and any NIC hardware manufacturer had to build dos-based network card drivers to support it too.
This sucks, and Microsoft finally relented. Windows PE is essentially their replacement for all DOS boot disk needs -- it can get your machine on the network and provide a 32-bit Windows compatible base for your various tools. It also provides a solid and much faster bsae for deploying other modern Windows OS'es.
When I first moved our company from DOS-based boot disks to Windows PE, I received a lot of negative response about the boot times. Common complaints were: "Dos took like 10 seconds to boot my R50 to the network; PE takes like two minutes! Why are we going backwards?" and "DOS did everything I needed to do, why aren't you using it anymore -- Windows sucks cause I can't do stuff that I usually do"
The reality was quite different, and it didn't take long for the technicians to see it. Pulling down our standard 2GB sysprepped XP image within DOS took about 15 minutes on a Thinkpad R50 with the gigabit network adapter, a gigabit network and only one logical (and gigabit-connected) network hop from the workstation to the hosting server (an HP DL380 with five 15,000 RPM U320 SCSI drives). Average speeds under DOS were ~230mb/min.
Windows PE on the exact same piece of equipment would pull down that image in two and a half minutes. Average download speeds under PE? About 1400mb/min. It doesn't take a genius to realize that a two minute boot time is more than forgivable with a 600+% increase in download speed. Upload speeds were also improved, on the order of 250% and more depending on hardware config.
It also paved the way for far more automation than was available in DOS, great examples: WMI queries for automatically detecting what HAL files to swap out on our XP sysprepped image, HTA application for launching our "build-o-matic" process that lays down our base application packages, far enhanced featuresets for automated ghosting to/from DVD/CD drives, offline defrag and checkdisk of NTFS volumes, you name it.
Preinstallation Environment is only getting better in Version 2.0, which is what you're using if you DVD-boot your Vista CD.