What is the purpose of DFT & IDFT blocks which are placed after the Analysis filter bank & before the Synthesis filter bank. The reason i am asking this question is because I tried to design Polyphase DFT filter bank & got the same output with & without DFT-IDFT block.
I guess you are referring to an implementation like the one shown here. If you leave out the DFT/IDFT blocks, then you don't have the modulation step which is necessary for actually implementing a filter bank, and you will not be able to do appropriate sub-band processing between the analysis and synthesis stages.
When you test your analysis and synthesis stages without sub-band processing in between you will obviously get the same result if you leave out the DFT/IDFT blocks. Analysis and synthesis together are then a pure delay and you might as well replace everything by a pure delay, but this defeats the purpose of the filter bank which is to split the input signal into sub-bands for further processing, and then to reconstruct the original signal from these separate sub-bands.