Using the Gas Buddy app is more than just finding the lowest prices on fuel. It's also about finding fuel...period. Many areas here in the west, one can drive many miles before finding a place to refuel. It's useful in that regard, particularly for pre-planning.
One area I like to go camping, it's about 30 miles in one direction to the nearest fuel, and 56 miles in the other direction. Veer off the main road for this area, and it's even more mileage to the nearest fuel. Helps in deciding where to top off the tank, depending on how far back in the boonies I want to get. This becomes even more critical if one is dependent on diesel fuel.