Just like trees, shrubs can grow rather large and disproportionate. Parts of them can also become infested with insects and disease, which means pruning is needed to keep them alive. Pruning also improves flowering. This is because pruning reduces the amount of wood within the shrub, allowing the plant to put its energy into the flowering process.
In addition, the shrub simply looks healthier and that is because it is. It will live longer due to being less likely to succumb to disease and insects. Pruning can also be a good safeguard against the heavy snow and ice that occurs during the winters.