Other posters gave good advice. Here’s why it’s good advice: energy refreshes. Energy refreshes allow you to upgrade characters faster, and you buy them with crystals. You get crystals from either spending real money or by getting rewards in arena and fleet arena. Going from rank 3000 to rank 200 or better in arena would get you double the crystals you’re getting now, and going from rank 200 to rank 50 or better would double your crystals again, allowing you to refresh energy much more often. Getting to top 20 in fleet arena also gets crystals.