When planting new shrubs in a garden bed, it is crucial to consider the mature size of the shrub to ensure proper spacing and healthy growth. Planting too closely together can lead to overcrowding, limiting air circulation, and hindering access to sunlight, which may stunt the plants' growth. By understanding the full-grown dimensions of each shrub, you can create a well-balanced, aesthetically pleasing landscape that allows for each plant to thrive and reach its full potential without competing for resources.