In the ideal perennial garden, something of interest is occurring most of the time but there can be occasional periods with little flower color. Although we can bridge these lapses with colorful foliage, we can also color the garden with flowering annuals.
There are, however, many other reasons to use annuals in the landscape. They create immediate impact, especially if installed in large masses. When the garden is used at the last moment as a stage for events such as weddings and parties, annuals and larger specimen tropicals will provide a colorful background.
The best gardens have a color scheme but it can be modulated from year to year with the use of annuals. For instance, using a blue and white scheme as the base, yellow and orange would brighten it considerably. The following year, pink and silver would soften it. Another year, red would strengthen it. Variety can be the spice of life.
Annuals are often used as an edging for beds but I would urge you to vary the depth of the edging in order to make it more interesting. Many annuals are used this way because they tend to stay compact and formal. Fibrous begonias and Sweet Alyssum are two of the primary choices as an edging but consider also curly parsley for its unusual texture or Zinnia angustifolia, a six-inch, bushy zinnia with masses of small blooms in white, orange, or gold.
There are many other ways to use annuals but this essay should start your thinking process.