Agreeing with teleview, the virtual channel numbers are often the only information a TV will display to you. If your TV is like most, you don't have a reliable means of knowing the real channel being received when there is more than one real channel carrying the same program feed.
A tuner such as the SiliconDust HDHR would give you both the real and virtual channel number, I find it's a valuable test tool as well as a good tuner. I'm not certain it's worth the $60 to $80 to you though.
http://forum.tvfool.com/showthread.php?t=820