A high level provides many advantages including the more ninjas that can be used in the team, the more new features are unlocked, the more buildings are unlocked etc.
The following are tips to quickly level up in gane pocket ninja tales of leaf (how to quickly level up)
Story
Use energy in the story to get lots of experience. the more energy used, the more exp you get.
Buildings
Create and upgrade buildings that can increase energy. Always take energy from buildings. And don’t forget to grab the free ramen.
Arena
Arena can give you a decent amount of experience too. Frequently the arena for farm exp
Add friend
Add friends and share energy
Finish dialy
Complete daily missions to increase the VIP Level. The higher the VIP level, the more opportunities to use energy-boosting foods.