Windows 2008 Server come in DVD format but not all the server hardware come with DVD drive. Don't be suprise that some server don't even come with CD/DVD drive (especially 1U and 2U server).
You might think that you can install windows 2008 Server simply by plug in USB DVD drive. Yes, you can boot up the Windows installation wizard but you might not be able to continue further because the DVD drive can not be detect by the installation wizard.