APK.CAP appears to target a specific pain point: intercepting, logging, and analyzing the network traffic generated by an Android application (APK) without needing to set up a full proxy environment (like Burp Suite on a rooted device). It likely streamlines capturing HTTPS/HTTP requests, API endpoints, and data exfiltration attempts directly from the APK.
If standard APK files work perfectly, why complicate things with .cap ? The reasons are practical, albeit niche. apk.cap