The main purpose of 1Blocker's in-app filter is to protect your privacy against in-app tracking.
While it may prevent some ads from loading along the way, not all of them can be blocked using this approach due to the technical limitations of iOS. For example, YouTube and Instagram in-app ads are impossible to block even with the in-app blocking tool enabled.
It can occasionally block some ads inside iOS apps, games, and mobile browsers, e.g., Chrome or Firefox, but only if they rely on trackers or attempt to connect to tracking-related domains.
Since targeting specific in-app ads is currently impossible, ad-blocking inside apps is a nice yet sporadic addition to 1Blocker, while its principal goal is to prevent in-app trackers from collecting your personal information.