The great English garden designer Gertrude Jekyll  was celebrated for her long, Impressionistic flower borders.  These planting schemes were based on the color spectrum, with monochromatic plant masses flowing seamlessly into plant masses of the neighboring color.

To keep these borders vibrant, Miss Jekyll relied on clever succession planting--organizing plant species and varieties so that one plant begins blooming just as another fades.

Even with a well-designed planting scheme, the great garden designer sometimes experienced gaps.  When that happened, she filled the "holes" with pots of annuals in the appropriate colors.  These were easily dropped in and just as easily removed as horticultural necessity dictated.

Most of us do not have Jekyll-esque color borders, but many of us experience gaps in our planting schemes.  Pots are the answer.  

Right now, for example, the spring flowers are blooming in my garden, but there are lots of bare spots that won't be filled until the perennials and shrubs leaf out later.  I have used pots of pansies and primroses to fill in those spots.  The effect is colorful and pleasing and the cost was minimal.

I even saw my neighbor peering enviously over the fence.  His grill may be bigger, but because of the pots, my garden is more colorful.