A few functional reasons to prune:

1. Improve the plants over all health - Frequently removing older stems encourages a plant to put energy into new growth, thus keeping the plant young.

2. Control or direct new growth - Each cut will stop the plants growth in one direction and redirect it in another, guiding the shape and size of the plant.

3. Prevent the spread of disease - Removing dead or damaged branches will decrease the chance of disease entering through dead wood and spreading throughout the plant.

4. Increase the number and quality of fruit, flowers and foliage - Pruning at the right time and in the right places can increase the number of shoots produced by the plant thus increasing yield.

5. Compensate for roots loss during transplanting - Plants have a balance between top growth and roots and when roots are lost in the process of digging an appropriate amount of top growth should also be removed.

6. Improve air circulation and allow light to reach inner and lower leaves - It is important to thin dense growth periodically to improve overall shape and health.

Pruning for looks is also a popular practice. There are many motives behind pruning to achieve a certain look. Here are just a few:

1. Introduce shape and structure to a garden - Pruned formal hedges of different heights and widths can be used in patterns or along borders to create beautiful effects.

2. Create a focal point - Pruning a plant to grow along a wall such as an espalier or creating a topiary plant sculpture are typical ways pruning can change the look of a plant and attract attention.

3. Expose colorful stems and bark - In many plants the some limbs can be removed when the plant is young to reveal decorative stems and colorful bark, especially in the winter.

4. Create more space for planting - Removing lower limbs can transform a shrub into a small tree, creating an elegant shape and allow more space for planting underneath.