I'm not sure about the ports on the 30 inches. But if they have two DVI ports it is understandable. DVI connections supposedly works better than VGA connections.
Ultimately, if you're using one monitor you will only need one DVI cable that connects your monitor to your graphics port, which has to be DVI as well. If you plan on setting up a dual monitor system, then you will need to connect another DVI cable to the second monitor from the graphics port. Your graphics card will need to have two ports to do this.
Note that if VGA can be converted to DVI, and vice versa. You will need a special cable to do it