The problem in recording video signals onto magnetic tape is that a video signal has enormous bandwidth in comparison to an audio signal. This means that the tape must move at a very high speed relative to the recording head.
Tape in an audio cassette moves at 47.625mm/sec (1+7/8 ips). To record analog video, the tape would need to move at about 5 - 10m/sec (11 - 22mph) depending on the required picture quality, which is impractical. The solution is to mount recording heads on a drum, which is set at a slight angle to the path of the tape and spins rapidly while the tape moves by slowly. The result is that the recording heads execute a helical path relative to the movement of the tape, and thereby write the video information at high speed as a series of diagonal stripes.
All modern videotape recorders use helical scanning, and the technique is also used to write CD-quality audio on to tape in DAT (digital audiotape) machines.