Each has its place.
In Time-Release plant foods nutrients are “coated” with a thin permeable membrane that allows either one or all of the nutrients to be metered out into the soil in a controlled manner.
Granular plant fooddisintegrates after watering and is absorbed into the soil. Nitrogen is metered out by biological activity in the soil.
With Water-Soluble plant foods the nutrients are immediately available to plants since they are delivered two ways, through the roots and through the leaves.