Hell or High Water seem to only be a simple heist story about two brothers (Ben Foster and Chris Pine). Their reason for robbing banks is to prevent the bank from taking their family ranch. While they are working the heist, a Texas Ranger who is about to retire (Jeff Bridges) hunts them.
Looking deeper, Hell or High Water is not a heist story at all. The heists are just a secondary focus in the movie and the real focus is on the relationships shared in the story. There are different dynamics between the different characters and this is what Hell or High Water is really about.
One relationship is the relationship between Toby and Tanner. Their contact is not based on their experiences together, and it even seems like their first experiences together is the spree of bank robberies they partake in.