Sometimes it’s quicker to frame the shot in a certain way to measure the exposure, then reframe the picture to take the shot. That’s what the AE-L button on the back of the camera is for. For a sunset silhouette, say, you could point the camera at the sky, press and hold the AE-L/AF-L button to lock the exposure and then reframe to shoot. The exposure will still be fixed for the sky.
The AE -L button lets you lock the exposure measurement while you reframe the picture.