In some markets, some TV stations air both digital and analog versions of the same channel on their HD feed....For example, CBS4-DT in the Quad Cities airs in hi-def on 58.1 (or 4.1 on converters) while the analog signal airs on 58.2 (4.2). Here's a suggestion for CBS4-DT and other station like this: You can do what two other stations do and air a weather channel--or better yet--an independent HD subchannel airing syndicated programming not cleared by any other station in the market! Now that's a good idea there!
As Neil says, this is probably done to provide a better feed for cable systems. It avoids the aspect ratio problems inherent in having the cable system simply downconvert the 4.1 HD signal. (for example, finding the branding "bug" spilling off the edge of the 4:3 standard-definition screen)
Since the first of the year, subchannels have carried various programming obligations - most importantly, EAS and children's E/I programming requirements. It's expensive to cover that, many stations dumped subchannels rather than buy extra E/I material. A downconverted copy of one's main program automatically fulfills both requirements.