If you have an object in a photo that is has been shot at some wonky angle and you want it to be straight, you can correct that. For example, I'm horrible at taking pictures of my needlework and getting it straight in the photo. Sometimes, I'm trying to avoid glare (and usually failing) but more often, I just don't have the camera at perfectly right angles to the object I'm trying to photograph.
Here's an example. This is a Teresa Wentzler design for counted cross stitch. Teresa's designs are legendary in the needlework world. Anyway, this darling hangs on my living room wall and I wanted to get a photo of it to put into a scrapbook. This wasn't the photo I used.
As you can see, it's a bit crooked. I was trying to avoid that reflection of the window behind me. I finally gave up and shot it at a different time of day. However, if this were the best that I could do and I had to use it, I'd want to have it straight.
I open the file in CS3, select the crop tool and draw a cropping box more or less around what I want. I don't have to be precise yet. Then I make sure that the Perspective check box (which is UN checked by default) is checked.
With Perspective enabled, I can move the marching ants around by the handles and position them precisely. They won't move the way they usually do, each corner can be moved independently of the others. I am not limited to a rectangle, I can make any four-sided shape I need. I take my time and get this exactly right. Time spent here will save me having to un-do and try again. If I need to make it larger, I press 'Ctrl' + '+' so I can see it. When I hve it just the way I want it, I press 'Enter.'
Here's my results. Still with that nasty reflection, but at least it's square.