It is a very good practice to put an RC filter on the power input of an op amp to prevent high frequency noise or stray RF from getting on to the opamp output, or preventing the op-amp from malfunctioning.
One of the previous comments said the power supply rejection ratio was 75db. Well this is only at very low frequencies. At higher frequencies the rejection gets less and less and may even approach 0db. So high frequencies go directly from the power pin to the op amp output. If the signals are strong enough, they can make the op amp offset voltage change, or even saturate the output of an op amp.
I am sure you are curious about how RF (radio frequency or high frequency) signals can get on a power line. This is very common during EMI/EMC compliance testing. In many parts of the world, you cannot sell a product with an op-amp in it unless you have proved by testing that the circuit will still work properly when injected with RF energy, or when cables to the circuit are immersed in an RF field.
In some testing RF signals are forced into the power lines through coupling coils, or are picked up by cables that are immersed in an RF field.
Have you heard the annoying buzz-buzz (217Hz) of a cell phone through a speaker. That annoyance can be minimized with this technique and other techniques.