If you pour hot water directly into the dry, unflavored gelatin, some of the granules will lump and not dissolve. This lumping diminishes the gelatin's thickening power and produces a detectable grainy texture in your prepared dish. First blending the unflavored powder with a little cold water softens and wets the crystals. When the hot water is then added, the moist crystals readily dissolve.
If you are blending at least an equal portion of sugar into the plain dry gelatin, you can forgo the cold-water step because sugar counteracts the clotting effect. For best result, however, the hot water should be under 180 degrees F.
** Asian Recipes