Shrub and flower beds add beauty to a home or building by helping to blend the vertical lines of the structure with the horizontal lines of the surrounding land.
Beds and plantings should be designed to help this blending effect without distracting from the natural beauty of the structure.
Some schools of thought on landscape design think there shouldn't be more than 5 different types of trees or shrubs within one viewing area. The belief is that too great a variety can draw the eye to the planting instead of the structure, as well as creating a "hodge podge" effect.
Other schools of thought think the greater the variety the better -- diversity adds to the interest of the overall planting. It is important to decide which style suits your personal tastes.
Most people find it desirable to have 'seasonal interest' in their landscaping, with evergreens bearing the brunt of the winter look, flowering trees and shrubs for spring interest, annual flowers for summer color, and varied leaf colors in the fall.