When you're working with a web site or a mobile app, you want to know how you are doing compared to the competition.
For the web, Alexa and Quantcast (or Comscore if you're selling ads) give good overviews of the relative sizes of web sites.
When comparing Facebook apps, Appdata is useful.
For iOS and Google Play rankings, Appannie is a good service to compare rankings.
While the data in each service can be a bit funky at times, that issue is usually manageable by looking at data over a period of time.