HTP increases the DR if you shoot in JPG. It underexposes by 1 stop of ISO, then lies about it in the metadata (that's why ISO 200 becomes the lowest setting - if you shoot at ISO 200 with HTP, the camera actually exposes at ISO 100). That 1-stop underexposure preserves a stop of highlights, the camera then applies a tone curve to boost everything but the highlights by 1 stop (meaning a stop more noise in the shadows).
If you shoot RAW, leave HTP off. You then have latitude to preserve >1-stop of highlights, and can apply your own tone curve.