There are a few reasons why you may need a retaining wall.
1. To create a nice level area which is to be retained with a wall. This could be used as a play area for your children. Or you may want other usable spaces or a patio or pool as well.
2. To terrace a slope with a retaining wall which you can use to garden on the flat surface.
3. The area maybe too steep to mow so a nice retaining wall can create a grade that is easier to work on especially when mowing and an area that is just easy to access.
You may choose from redwood, concrete, block keystone bricks or any other kind of retaining wall you have in mind.