Gragas has one of the best and most versatile ultimates in the game. Some of the main uses are for scattering and displacing enemies to secure kills for either you or your teammates, however there are many other uses.
1. You can bait enemies near tower and then ult them and knock them into tower range if they poke you. Follow that up with your E-Q to secure a free kill, even without your ADC present.
2. You can use your ult to spread enemies from contested objectives such as Baron or Drake. This can help your jungle steal the objective with ease. Typical objective engage would be to E through the back objective wall after warding, and then ulting as needed.
3. You can kite around the map and ult to stop enemies from chases or engaging on certain vulnerable allies.