All other fruits are tainted fruits (partially eaten fruits). For example, the apple tree grows from the seed of another eaten fruit and that fruit is treated as tainted.
In the case of coconut and banana, the shell or the sleeves is not used for anything. To grow a coconut tree, you have to sow the entire coconut itself and Banana tree grows with the sampling. This is the reason why Coconut and Banana has an important place in all religious activities. Our Ancestors had found this reality long ago and they had made it as a system which is still followed religiously.
The fibre covering of the dried coconut is removed except for a tuft on the top. The marks on the coconut make it look like the head of a human being. The coconut is broken, symbolising the breaking of the ego. The juice within, representing the inner tendencies (vaasanas) is offered along with the white kernel - the mind, to the Lord.
A mind thus purified by the touch of the Lord is used as prasaada ( a holy gift). In the traditional abhishekha ritual done in all temples and many homes, several materials are poured over the deity like milk, curd, honey, tender coconut water, sandal paste, holy ash etc. Each material has a specific significance of bestowing certain benefits on worshippers. Tender coconut water is used in abhisheka rituals since it is believed to bestow spiritual growth on the seeker.
The coconut also symbolises selfless service. Every part of the tree -the trunk, leaves, fruit, coir etc. Is used in innumerable ways like thatches, mats, tasty dishes, oil, soap etc. It takes in even salty water from the earth and converts it into sweet nutritive water that is especially beneficial to sick people. It is used in the preparation of many ayurvedic medicines and in other alternative medicinal systems. The marks on the coconut are even thought to represent the three-eyed Lord Shiva and therefore it is considered to be a means to fulfill our desires.
Coconut and Banana are the only two fruits in this world which are considered to be “Sacred fruits”.