you get treasure maps which are materials required for evolving your cards. the 25 energy fields give you red, green and blue treasure maps and void field gives you gold, purple and rainbow maps -> this is rather difficult if you're a beginner
If you're low level, I wouldn't focus as much on the maps as leveling up. Most maps help evolve cards to 6* and beyond. Unless you have immediate goals of gem recruits, your energy is best served leveling so you can get your command points up first, which is key to using those evolved cards.