There are probably 12 gas stations between my work and home. I can quickly check GasBuddy and see which one is the lowest and stop there without going out of my way. Sometimes my Costco is 10 cents/gal less that the others (usually just after all the other stations have done an increase), but usually the wait there is only a couple of minutes at the most. After prices have stabilized, Costco is usually within a penny or two of the others.
The other reason GB is handy is if you are traveling and don't know where the stations are. Typically, if you're on the Interstate the stations right at the exits are more expensive than the ones a couple of blocks farther. Of course other apps like Road Ninja are good for that as well.