Wooden pressure treated stakes and posts are useful for supporting low level runs of railway sleepers and holding them in place on the ground. Most people knock the wooden stakes into the ground between 150mm and 300mm, and then screw the railway sleepers to them. The wooden posts are only 600mm long and 50mm x 50mm wide, so they are only suitable for holding up to 300mm high railway sleeper projects that are not holding back much weight. If you are planning a raised bed or wall that is much higher and more substantial than this, you'll probably consider using cut lengths of railway sleepers, that are concreted into the ground vertically, and will act as much longer and more sturdy and durable retaining posts.
Alternatively, wooden stakes are also great for marking out the layout of your grand design so that you can visually see what your masterpiece will look like! You'll see them everywhere on buiding sites where they are using them as marker posts.