Optical products with variable magnification are more versatile as they're designed with a wider range of magnification and larger viewing angle. For example, an 8x56 rifle scope has a fixed magnification of 8x and a fixed field of view of 5 meters at 100 meters. For comparison, a rifle scope with variable magnification 3-12x56 has a more versatile magnification range for observing the whole surroundings due to the big field of view (at 3 times magnification) and in the same time a detailed evaluation of the observed object (at 12 times magnification).
With their high number of lenses in housing, optics with variable magnification are larger, heavier and have a lower permeability of light. Variable magnification is rarely present in binoculars, but it's mostly used in rifle scopes and spotting scopes. Advantages of the variable magnification regarding usability with rifle scopes and spotting scopes outweigh disadvantages – in short, one fits all. High-quality variable binoculars are very rare.