ok if you went into Gimp and made a circle... it looks all smooth right? Well, zoom in.. see them squares? Raster images are made up of wee little squares. Because of that they will lose quality if you resize them. Vector images don't.
For example...
If I made a logo for the Dome in Gimp. I make it 400 pixels by 400 pixels (pixels are them wee squares)... but then, I want the logo on a big sign.. and I need the logo to be about 2 feet x 2 feet... I think you see the problem.
With a vector, I can scale the logo from 1 pixel to something to fit the SkyDome and it will be the same quality.