When using your DSLR, do you have problems using the half-press shutter button to first of all focus and then press fully to take the picture? Many people do. For whatever reason they are a wee bit heavy fingered and find it difficult to control the half press and end up taking the photograph instead. This means that when the camera is auto-focus mode, the camera has virtually no time to focus and will not necessarily focus on what you want it to focus on.
There is a way round this. Check out this video.