The DocType, has recently become uber important as tells the device what set of commands to use when displaying the 'webpage'.
In this case the html 4.0 transitional document model is specified, the key role at moment, for me at least, is creating 'websites' that look identical on multiple different devices, eg IE, Firefox, Digital TV, mobile etc.
If you google browser compatability chart that will help you visualise the differences in the document models.
Hope this helps
D