If we throw away the seeds after eating the other fruit, it'll grow as a plant again. But if we throw the coconut shell after eating it, if we throw the banana either removing the outer skin or without, it'll never grow again. This represents the Mukti/Moksha state which is that the ultimate state of soteriological release, the liberation from repeated rebirth. Thus we are offering these to wish God to offer us the Mukti state.
This is also little almost like above one. Since coconut and banana can never come from the seeds thrown after eaten by us, they're considered pure with none contact of human saliva in any ways. Hence they're offered to God because the purest fruits.