hi gurpreet,
you might be thinking that as the CP is discarded at the receiver, the CP can be anything. yes, the cyclic prefix can be anything. But, it is more advantageous to have it as a repeated version of the starting portion of the same OFDM symbol, this will help in maintaining the cyclic property of DFT so even if there is a slight error in detecting the start of OFDM symbol, this CP preserves the cyclic property of DFT.
If you remember, DFT assumes that the input is periodic when it is not the case. This has a direct link to the concept why CP is what it is..