AE Lock is for situations where you want to use metered exposure later than you metered it:

  * locking exposure capturing parts of a panorama - for seamless stitching, frames with similar exposure will work better than differently exposed ones;
  * when you meter from a gray card, lock exposure, then remove the gray card and compose;
  * you point your camera on a surface that should be exposed as 18% gray, lock exposure, recompose