purpose of fft
Your question is really strange for me. I supposed all participants of this prestigious forum are aware of Fourier transform and such questions will never arise in their minds. It's difficult to answer this question. Take any book on DSP (I insistently recommend you Oppenheim "Digital Signal processing"), read it very carefully and pay attention to the paragraphs, devoted to Fourier analysis. Try to understand the sense of applying Fourier tranform in Signal Processing. If you succeed in this, your question will be solved automatically.
If anything won't be clear, let me know. No doubt, I'll help with great pleasure.
With respect,
Dmitrij