Imagine that there are plenty of gull. In the universe the game is located in, gulls just pop-up with each level-change.
You, the player of the game, are supposed to get their numbers down by using advanced weaponry.
The game can be sustained for unbounded periods of time. The goal of the game is to shoot gulls.
You can shoot a lot, or you can shoot a few, it does not really matter. They keep re-spawning anyway with each level change.
The game lacks any form of moral. It is an unfair struggle between you and those gulls.

Your weapons have been modified to accommodate arcade mode. That means no running out of bullets and thus no reloading.