I like retouching my photos before I share them with anyone. My mother-in-law thinks I fuss about it too much but it's just me. One of the first things I learned doing image enhancements using Photoshop is manually adjusting levels.
Yes, it's easy to just use the Auto Levels function but there are times when the results just doesn't come out right. Then again it's just a matter of personal preference... and time constraints.
I'm nowhere near an expert. I'm just your average user, giving you my two cents on what works for me, and probably would work for other average users as well. I haven't really explored the nuts and bolts of the Levels adjustment because I haven't had the need for more than what I have been doing now. At least not YET.