It can be entirely idiosyncratic, or at least apparently so. Such bouncing reflections can make your Mac go weird. Terminators are devices that stop the bouncing signals. In practise, my Microtek scanner works just fine with or without any terminator installed. As I said, suggestions, not rules, apply.
Most SCSI devices come with either an internal or an external terminator.
A few upscale SCSI devices have active termination, which is very convenient circuitry that turns termination on or off automatically as needed. Take anything either say with a grain of salt. Long SCSI cables are often troublemakers. The tiny internal wires can fray, jacks can become damaged, and shielding can rupture. Generally, the thicker and heavier the cable insulation and sheilding, and the more robust the connectors, the more reliable a SCSI cable will be.