When do I use the HDR function on the iPhone 4s camera?

Just wanna no in what situation would I turn it on

When there's a bright back light, use it. It will bring out more detail in the darker areas and make the light less extreme. Also, when you are taking pictures of landscape if can help. You can turn on the option to take a normal pictures and an HDR picture. Then you can compare and get rid of the one you don't want. Just don't use it when the thing you are going to photograph will probably move. Also, if can sometimes make colors less vibrant. Just compare between normal and HDR.