There are 2 main strategies imo.

1. A blog that makes you a thought leader. People will like you, check out what you're doing, and perhaps start using your app. This is the typical way to go for b2b, or if you have a big ego :) Another pro is that if you change your app, you still (may) have your readership.

2. A blog that extends the content of your app. This fundamentally is a set of landing pages that improve your SEO. One common strategy is to talk about problems that people may have, and that your app solves. Whether you want or not to be too explicit about your app it's a matter of tastes.