Take look through the list of functions. Basically these are the functions that Windows is using to draw itself. They extend the functionality of Blitz. This is what MicroSoft say...
"A graphics device interface (GDI) enables applications to use graphics and formatted text on both the video display and the printer. Microsoft® Windows®-based applications do not access the graphics hardware directly. Instead, GDI interacts with device drivers on behalf of applications. GDI can be used in all Windows-based applications."
For example I wanted to get the desktop colour depth, there is no way to do this in Blitz however GetDeviceCaps returns this plus other exciting stuff like version of the display driver, width in millimeters of the monitor (!!!!).
This is just one function in a list of approx 500 (including User32.dll and Kernel32.dll) that can do pretty much ANYTHING that you could EVER possibly imagine to do with Windows.
For a detailed breakdown of GetDeviceCaps see here...
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/gdi/devcons_88s3.asp