In order for two computer systems to transfer data between them the information has to be wrapped up at each layer, transmitted across the physical wire or with radio waves. On the receiving side each layer will be unwrapped in turn so that the correct process gets the right information into the correct place.
There is no guarantee that any packet will get across the physical media. So at level 4 and above you can manage the transport layer to ensure that the receiving computer has gotten all the information you are trying to send, or at least you will know the data was not received.
Most applications will also send acknowledgements across the connection so that the application can be sure that the data has been placed into a database on the other side of the connection.
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