Hi confused, I'm Sora! Lol
In all seriousness, the purpose of collecting burgers serves the same purpose as coins in Super Mario Bros, in which you get 100 and you get an extra life. The enemie shoot you because they don't want you to get the legendary burger treasure! Also, after you defeat the last captain, you're free to explore the other levels!
Also, thanks for bringing the Instructions thing to my attention, I'll fix it. Thanks for playing, though! I was honestly really surprised anyone was even interested in playing this game, it's something I just sorta threw together for the game jam, and it isn't exactly my best work. Anyway, happy gaming! ^_^