Sensor Tower is an App Store Optimization platform that I use to help find the best keywords for my apps. Search has always been a huge part of app sales, but with the iOS 6 app store changes it is even more important. It is also significantly harder. Before, users got a page of apps with each search and it just took a good icon to stand out. Getting in the top 5 was enough to get you good visibility, and getting in the top 25 wasn't a terrible outcome.
Now, the search results only show 1 app at a time, and you have to swipe each time you want another app. I suspect people rarely get to 25 before making a decision now, and being 5th is much less exciting than it used to be.
There are a few things that you need to know when working with your keywords that Sensor Tower helps out with. You want to know how much traffic the keyword gets, how hard it is to rank for that keyword, and what your rank is for it.
This is a screenshot for some of the keywords for an app of mine (Walk Tracker). It lets me know which keywords have more traffic, and how hard they are to rank for. GPS and jogging get a lot of traffic but they have significantly more competition. Walk tracker is much easier to rank for, and because it is in my title I happen to rank pretty high for it.
Part of what this shows is that I need to work on some of my keywords and/or focus more on the long tail. I don't rank very highly for gps or fitness, but I'm #5 for "gps fitness tracker". You can use this tool to find keywords and phrases that get decent traffic but that are easier to rank for. Optimizing this is really key. I've had keyword changes in the past that meant huge spikes in revenue for me. Getting visibility on the store is huge.
If you are having trouble coming up with keywords, Sensor Tower also gives you the ability to spy on your competitors likely keywords as well as offer suggestions to optimize your keywords.
I'm not entirely convinced this service is something that you need to pay for forever for each app, but it is definitely something worthwhile to use when launching and updating your apps until you are happy with your keywords and doing well on the store. There is certainly value in tracking your rankings over time and comparing them to your revenue though.