Camera entities are quite useful. They are mostly utilized in TrackView sequences, but can also be used for pre-caching locations or making screenshots. Placing cameras is simple, since a camera entity can just be dropped into the level from the RollUpBar.
A nice and very well hidden feature is the ability to manipulate the camera target independently from the camera itself. Usually, when placing a camera entity, Sandbox will not create a camera target entity for you to work with. If you would like to have access to the camera target, you have to create the camera entity the following way:
Left-click on the camera entity in the Object/Misc section of the RollUpBar.
Move the mouse over your level terrain and click and hold the left mouse button.
Move the mouse and release the left mouse button to place the camera target in the level.
Quite well hidden, isn't it? However, once you know the trick, it becomes easy to create cameras with a separate camera target...