Auto Exposure is very useful, because if you are photographing on aperture priority, or shutter speed priority, rather than having to reajust the cameras programming every time you change the scene, you can just allow the camera to automatically change the scene as necessary. Also if you enter a value that the camera considers too high, or too low, it changes it automatically.

Auto Focus is also extremely useful, for example if you are shooting in macro, but then choose to move to shooting in normal, all you have to do is depress the shutter button half way and the camera will focus of its own accord, despite being programmed to shooting in macro mode.

BUT, if for example you are photographing images that you will later put together as a panorama, you need to stop the camera using AE, because the panoramic stitching software will have trouble stitching the images together smoothly if the exposure is different in each frame. Also, if you are on shutter speed priority, and taking action pictures, you need to make the camera shoot at the shutter speed you choose rather than that that it thinks is more suitable. There are also many other reasons why you might want to overcome this, but is it possible?

The answer is a big YES. As I have explained above, AF and AE are very useful for day to day photography, but there are times when you will want to override this mechanism.

Camera manufacturers have long since acknowledged this problem, and have created the AE-L/AF-L button, also displayed as AF Lock, AF/AE Lock or AE lock. This button locks the AE and/or AF, for successful shooting

Auto Focus Lock is also extremely useful, for example, if you are shooting in macro, and want to remain in that mode, despite your subject requiring normal to be in focus, the AF lock button allows you to lock AF so that it remains in the mode you have chosen it to be in.