multicycle path for relaxing design

Quartus integrated synthesis does not make use of timing constraints. However, setting "Optimization Technique" to "Balanced" might still give you a good combination of area and speed.

The Fitter does use timing constraints. Using the multicycle exceptions will let the Fitter know it does not have to work as hard on the paths with the relaxed requirements. That is probably what the Synplify documentation was referring to for Quartus. Multicycle exceptions can let the Fitter focus its effort on the paths that truly are most critical. If you are using physical synthesis, that could prevent the Fitter from needlessly duplicating logic for these paths.