Servo tracks are written on the back of tapes to keep the read/write heads on the correct data track.  Newer tape media have very thin dense data tracks.  256, 384 and 768 data tracks on a half inch wide tape are now common.  While moving from one reel to another when loaded in the tape drive, there may be some lateral tape movement that could cause the drive to 'lose track' of which data track it is reading.  By putting servo tracks on the back side of the media, where it will not affect the actual data being stored, the servo system of a tape drive can tell precisely which track is being accessed and allows the tape drive to accurately position the tape media.    TapeLady