I think the VGA box provides a much higher quality signal to the monitor. A TV Tuner card would have the signal get converted to composite(RCA) or even RF, then converted by the tv card to a signal your monitor could interpret. Probably higher quality than a TV because of the resolution of a monitor.
But the VGA box takes the much higher quality signal from the DC's multi-out and sends it straight to the monitor. As far as I know, its the highest quality video out for any game console out now.
It all boils down to the multi-out on the DC being able to output many levels of signal depending on the connection.
Warning, though - not every game supports it, but some 3rd party (non-sega) ones can fool many titles into using it. But still not all, I think.