The ability to see how my site performs in comparison with other sites of similar size, traffic etc. I like that it also shows me where things are slow or not loading correctly. It can be very difficult at times to understand what it means for a particular page load time - but there's usually enough information given to help figure out why something might take longer than expected. There are many different ways to compare websites, so this feature has been helpful in finding issues we were unaware of before using it. We've found that our site loads faster when compared against sites of similar volume (ie similar number of visitors).