The OBB file is critical because the APK file alone usually only contains the basic code to launch the app. For DLS Classic, the OBB stores the player models, textures, and stadium data. Without it, the game will often fail to load or display a "download failed" error upon opening.
Installing a classic game manually requires moving the OBB file to the correct folder on your Android device. Here is the standard procedure: dream league soccer classic obb file