A clumping plant expands slowly outward from a single crown and stays where it was planted. It is the opposite of a runner, and the difference shows up three or four years in — one of them is still the shape you planted and the other is in the lawn.
That predictability is the whole value. A clumping plant can be spaced to a plan and the plan still holds in year five. It does not need edging to contain it, it does not come up where it was not wanted, and it does not have to be pulled out of its neighbors every spring.
The trade is patience and cost. Clumpers fill a bed slowly, so a planting that will look right in year four looks sparse in year one, and covering the same area takes more plants. On ornamental grasses in particular this is the distinction worth paying for — a clumping grass is an asset and a running grass in the wrong bed is a long-term problem.
Most eventually want dividing, when the center of the clump thins and the growth moves to the edges. On most perennials that is somewhere between year four and year seven, and it is routine rather than a failure.