I get them, and delete them.

It would probably be similar to the Google in Your City events, where an AdSense worker discusses ad placement and color issues with you. That kind of thing. It's easier to critique someone else's site than it is to critique our own. That's the value. Good info sometimes.

I always accept them and sometimes even request a review like that.

The advisors vary in their levels of helpfulness, but it never hurts to hear them out. One time, a very small change suggested from one of these reviews resulted in a significant income boost. And it was not something I would have thought to do on my own. And another time, it saved me from placing an ad in a policy violating area. Again, not something I thought would violate policy, so I was really glad it was pointed out to me.

Now, if I make any major changes to placement or display of my ads, I ask if an advisor will take a look. I don't always do as they suggest (some of them still roll with the "more ads, higher placement on page" advice - which is silly) but I always listen.

Beyond that, just scroll through this forum and count how many "I got banned and don't know why/for wrong reason/for no reason/I fixed it but they won't reinstate" posts you count. While the advisor may or may not be able to help you in those cases, having an actual person to contact in case of emergency is better than an automated email.

One time, a very small change suggested from one of these reviews resulted in a significant income boost. And it was not something I would have thought to do on my own.

I had the same experience as hannamyluv, and it meant a big boost to my earnings over time as well. You have absolutely nothing to lose from talking with an official Google AdSense representative, and perhaps they'll provide some tips that'll help you too in a big way. I've had a few calls with them now, and have attended one of their AdSense In Your City events, and I've always gotten something valuable out of those experiences.

Thank you for you answers.
I just wondering what small changes could have impact on earnings. Increases CPC, CTR or something else?

I always accept them and find them useful. Let them tell you what changes might have an impact on earnings.

I always listen, don't always implement, but I always listen. At one point (like others) AdSense predicted I would get a significant income boost if I modified my layout just a bit and used a different sized ad. The increased earnings were noticeable within the hour, and it ended up performing even better than they predicted. Your mileage may vary, but it's always worth a listen.

I just wondering what small changes could have impact on earnings.

You'd be surprised. In my case, it was simply moving the ad up about 200px on the page to be closer to social media icons. Not in a tricky way, just side by side instead of stacked. Was not even something I thought about being a focal point for people's eyes.

As Netmeg noted, the results were noticeable within the hour.