You're in control - if you choose to be!

By controlling your aperture, you control how much depth of field you get.  This is probably the most common way of taking pictures.  If you are taking a scenic picture with a foreground and a background, you will often want ALL of it to be in focus.  But if you're taking a subject that you want to stand out from its background, you will probably want to limit your depth of field - a very good technique for taking a portrait.  Almost every picture I take is taken in "aperture priority" mode because most of the time I want to control the depth of field.

In this image, a large lens opening combined with a telephoto lens caused the background to go out of focus and makes the lilacs really stand out.

 

In this image, there is a foreground and a background that I want to be in focus.  So I used a small lens opening with a wide angle lens.  The result was that almost everything in the picture is in focus.