DVD layout is divided into data blocks containing 16 sectors. When there is read error in certain sector then also all 16 sectors in data block is not readable. You should enable option to skip 16 sectors if there is read error. Trying to read each next sector is just waste of time if data block is already not readable. It'll be much faster to recover bad media with option to skip 16 sectors in case read error occured on sector divisible by 16.
Edit: More info about ECC data blocks here - http://www.lightbyte.com/DVD_Sector_2.doc
