"Auto Lighting Optimizer" is Canon's term for in-camera post-processing to automatically adjust shadows and highlights in high-contrast images. Nikon has a similar feature called " "Active D-Lighting", and other makers have corresponding capabilities as well.

These all do the same thing: they adjust the tone curve so shadows are brightened and highlights roll off more smoothly. The exact function is "secret sauce" — you don't get much control over the exact effect. It's very much like using a software program which can adjust this curve and hitting the "auto" button. Sometimes you get good results, and sometimes... not so much.

The histogram is basically unrelated, although if you see that your scene smashes up against both ends, that's a good indication that using this feature might help.

Now, as for whether "pro photographers" use this: sure. Some definitely do. Professional photographers do many different things, have many different approaches, and correspondingly have very different needs. This doesn't have very much impact on whether the feature is good for you, even if you are a pro yourself or aspire to be one.

For some pros, getting decent results fast is the most important. They're trying to make a living, not art, or perfection. Shooting in JPEG and using the camera's JPEG capabilities to the fullest is very convenient here. For others, meticulous control is important; these pros shoot in RAW and hand-tune everything. Others fall into the spectrum in between.