Other reasons include having full control (rewind, step, pause), HQ scaling in a convenient window, or even for a desktop background (i.e. borderless maximized window).
Which filters are in use? Default should be Color Space Converter so if not could specifically add it from External Filters and set to Prefer or otherwise Reset defaults from Miscellaneous. Optionally Export first so can easily import your current settings again -assuming using registry and not INI file option.
Ruled out a bad source and tried to mess up playback with scaling and distorting aspect ratio but could not with madVR nor EVR. Using MPC-HC 32-bit on W7 64-bit. Maybe you are using MPC-HC 64-bit or it is a video driver issue?