Some perennials and biennials are grown in gardens as annuals for convenience.
Annuals are often used in gardens to provide splashes of color, as they tend to produce more flowers than perennials. Also, many food plants are, or are grown as, annuals, including most domesticated grainss.
The life-cycle of an annual can occur in a period as short as two or three months in some species. Many garden weeds are annuals.