eWhisper - 12:50 pm on Feb 20, 2013 (gmt 0)
There are usually three reasons I'll block IPs:
1. No matter what the settings I choose, I keep getting some visitors from countries I don't want to target. This is most common with international accounts; so I'll use IP blocking to block large ranges of country based IPs.
2. I have some competitors that try to click on the ads all the time. In these cases, I'll either block their IPs, or serve them a different landing page than what everyone else sees.
3. Its a large company and the sales or support staff often search Google for their own company to either see the ads, or to navigate to the products and they click the companies ads all the time. In this case, I'll sometimes block the company from seeing its own ads and then teach them about using the ad preview tool to see ads.
I can't say that I use this feature a lot; but there have been times when I have found it useful.