Because the scaler in some panels may be inferior. Also, not all flat panels are 1366x768. Some are 1280x720 and then, if the 868 scaler does a better job than the inbuilt one, it may be worthwhile setting the 868 to handle the scaling.
"Scaling
Being fixed-pixel devices, like plasma and DLP displays, LCDs require manipulation of every video source signal in order for these signals to be displayed at the panels "native" resolution of 1280x768 or 1280x720. Lower resolutions from cable, satellite, DVDs, VCRs, and camcorders must be upsampled to the native resolution; the highest-resolution HDTV images must be downsampled. As with plasma and DLP devices, the quality of the scaling has a direct impact on image quality. But good scaling is complex and expensive; the less expensive the product, the lower the scaling budget. "