From my perspective, annuals' best feature is that they bloom prolifically for a LONG season. Since they only have one year of life, they continually produce flowers. They can also be divided and otherwise increased with simple gardening techniques. So a perenniel is a long-term investment which will reliably produce beautiful displays. Perenniel also produce the showiest and most fascinating displays.
Most gardeners interplant annuals among perenniels to fill in colors.