Probably A/B testing new features. I have noticed that many subscription-based apps now tend to update weekly or bi-weekly, so maybe it has to do with checking subscription renewal. These apps also tend to include very generic release notes: "Bug fixes and performance improvements, we update the app frequently to bring you the best experience"... without really telling what was fixed or improved, giving an impression that you are using a buggy software which requires constant fixing.
Outlook, VSCO, Word... just to name a few apps using this practice